Feel free to downgrade this to Enhancement if appropriate.
The "install" script, as used by the Windows installer (and by me in an unsupported mode) saves the old jetty-base during an upgrade.
One directory inside jetty-base where that's not a great idea is jetty-base/tmp. As I understand it, this contains nothing of value but may be quite large. When updating to snapshots, I've collected one that runs around 350MB and another one that is 40MB.
Each subdirectory of tmp seems to be around 40MB. The culprit seems to be a webinf/WEB-INF directory of about 37MB. Possibly an unpacked version of the WAR?
It's possible that these should be cleaned up in the normal course of events and that I'm mistreating the IdP somehow to have multiple copies left around, but there's no reason to save even a single copy as far as I can tell and it would probably be best to wipe the tmp directory (or the saved copy of it) during an upgrade.