A full Cygwin installation can range from 23 to 112 GiB, depending on how you define "full."
Your 100 MB number tells me that you just clicked through the defaults presented by Cygwin's setup-*.exe program, selecting no optional packages, because that installs only the Base package set, which currently amounts to 0.1 GiB. Cygwin follows the modern net-connected software distribution model: it assumes you can just run setup-*.exe again and select new packages as you need them.
The Cygwin maintainers try to keep the Base category's package set as small as practical.1 A Cygwin Base install gives you something much like an old-style Unix installation, covering little more than what POSIX specifies.
So How Do You Get a Full Installation?
The Cygwin installer does not have an obvious way to get a "full" installation, on purpose, because no one needs literally every package in the Cygwin repository.2
There is, however, a sneaky way to install everything. At the Select Packages screen...
...switch to Category view, then click the "Default" text to the right of the "All" group header. It will change to "Install," as will the corresponding text in all of the groups underneath it. This marks everything for installation.
I include this tip for completeness only. You do not want to do this! It will install gigs and gigs of stuff you will never use. Currently, there are 11242 packages in Cygwin,3 and installing every last one of them took 93 GiB of disk space for the installation tree plus 19 GiB for the download tree? the last time I tried it. That gives the 112 GiB upper limit above.
All that unused software carries several costs. Even if disk space, download time, installation time, rebasing time, and local bandwidth use are of no consequence to you, consider the generous free mirror's wasted bandwidth.
I've done the experiment, so now you don't have to.
An Intelligent "Complete" Installation
I've come up with a simple set of package exclusion rules that results in a much smaller installation:
Skip all of the -debuginfo packages. Few people need these, and they take up a lot of space. Savings: about 53 GiB in the installation tree alone; more in the download tree.
It's easy to apply this rule. After selecting all packages for installation with the sneaky trick above but before you move on to the next screen, click the "Install" text next to the "Debug" category header until it switches back to "Default."
If you've already installed the debug packages, click that text until it says "Uninstall" instead.
Do not explicitly install any of the lib* packages. Let Cygwin's setup-*.exe automatically install libraries to satisfy package dependencies. Savings: about 5 GiB ?
To apply this rule, switch the "Libs" category to "Default" or "Uninstall" as you did with the "Debug" category. The installer will figure out which libraries you actually need in a later step.
Skip the cross-compilers and associated packages. Again, few people need these.? Savings: About 4 GiB
There are two major sets of cross-development tools in Cygwin: the set for creating Cygwin executables of the other word size (i.e. 64-bit tools and libraries for 32-bit Cygwin, or vice versa) and the set for building MinGW executables of the same word size as your Cygwin installation.
To apply this rule for a 64-bit Cygwin installation, while still on the "Select Packages" screen, type cygwin32- in the package name search box at the top of that screen, then click the Default text next to each top-level category until it cycles to Default or Uninstall, as above.
Repeat that for mingw64-.
The idea is the same for 32-bit Cygwin, except that you search for and exclude packages with cygwin64- and mingw32- in their names instead.
By following this rule set, I was able to install nearly everything, taking only about 23 GiB.
Paring That Down
We can get the installation to be even smaller by excluding several other notorious disk hogs:
X11, the desktop environments, and the GUI apps together require about 11 GiB.?
A Cygwin Base + Devel installation comes to about 10 GiB.
A Cygwin Base + TeX category installation takes about 5 GiB. If you install only your native language's support package, it comes to about 3.7 GiB instead.
All of the -doc packages combined chew up about 5 GiB of disk space.
Someone who isn't a software developer, who doesn't use Cygwin for GUI stuff, who uses the Web for docs, and who doesn't use TeX for document creation could thus have a "full" Cygwin installation in only about 1 GiB.?
If you use Cygwin the way it is intended to be used, installing the base and only the extra packages you need at the moment, you probably won't even get your installation size to even those levels. I use Cygwin this way, and my installations are typically well under 1 GiB, yet they are "complete" by my lights, since they meet my current needs.
For Extra Disk-Filling Fun...
All of this testing was done with the 64-bit version of Cygwin. You can roughly double the above space requirements by installing a parallel 32-bit Cygwin installation.?
Doing so is not pointless. It is a viable alternative to using cross-compilers, for one thing. For another, the fundamental incompatibility of the two Cygwins means you may have need of both.
Footnotes
There are occasional threads on the Cygwin mailing list where someone argues that some very common package should be included in the Base category, such as Perl, and the result is usually that the maintainers decide not to add it to Base.
You also occasionally see the opposite: some package accidentally slips into Base, usually via an incorrect dependency. Shortly after the problem is brought to the attention of the Cygwin maintainers, the status quo ante is restored. (example)
Perhaps I am wrong.
There could be a vision-impaired Czech immigrant musician who completes US government software development contracts on the side while not busy brushing up on his technical Hindi vocabulary by translating electrical engineering reports into his adopted Mandarin.
I want to meet him. He sounds like an interesting guy.
Plus, I think I can help him with his plan to create a Tcl/Tk GUI front end for Orpie. Naturally, I will try to talk him into porting it from Ocaml to C++/Qt.
I mean, Tcl/Tk, seriously? In 2018? Let's not be ridiculous.
curl -s https://cygwin.com/packages/package_list.html | grep -c x86_64/
Cygwin's setup-*.exe doesn't delete the downloaded package files after installing them. This is useful at sites where you have multiple Cygwin installations, since you can put the download directory on a shared network drive. Each package then only has to be downloaded once at that site.
My 105 GiB upper limit assumes you will download and install to the same drive, and that you will keep the download tree in case you need to reinstall it later.
Not only does setup-*.exe not delete downloaded package files after installing them, it doesn't auto-purge old versions, so your download tree grows and grows over the years you use Cygwin. (There are scripts floating about the net to solve this problem, such as this one.)
All data storage values given in this answer are apparent disk usage numbers — du -bhs — rather than actual disk usage numbers, which would account for the file system overhead, since that varies between systems. This affects the installation tree to a much greater degree than it does the download tree since the proportion of small files is much greater in the installation tree. Expect something like +1% in the download tree and +5% in the installation tree.
You may wonder why there are libraries in the Cygwin package repository that you don't need even though you've installed "all" packages. There are several reasons:
some libraries are obsolete, but are still present on the mirrors
some libraries come in multiple alternative forms, so that people who know they need something other than the default can choose it
some libraries are there only for people writing their own programs, not to support any existing Cygwin package
Pretty much the only people who need the Cygwin cross-compilers are the people maintaining Cygwin packages, since maintainers are expected to build for both 32-bit and 64-bit Cygwin unless there is a good reason not to.
There are probably more people with a good justification for MinGW cross-development tools, but there's also the option of using MinGW and MSYS instead of Cygwin. Also, I am guessing that the number of people who do dual-stack Cygwin + MinGW development is smaller than the set of people who use one or the other exclusively, or nearly so.
It is not easy to do this exclusion, because GUI packages are scattered throughout the Cygwin packaging system, and th
