Audacity development source code is on GitHub:https://github.com/audacity/audacity
For a one-off download, click on the "Clone or Download" button on the right side of the page, then select the "Download Zip" option. The file size is 26.3 MB.
For developers that want to be able to keep up to date with the current code it is better to clone the source code repository.
Release version source code is available from this page:https://github.com/audacity/audacity/releases
Windows users will usually want the most recent (2.1.3) Zip version (28.4 MB)
For Linux, there is a "mini" tarball (7.7 MB) available from the link at the top of this page on the Audacity website: http://www.audacityteam.org/download/source/
Shared libraries that are usually installed as standard on Linux Desktop systems have been removed so as to reduce the download size.
See also this page and pages that it links to: http://www.audacityteam.org/download/source/