What compression method does the non-universal, self-extracting zip format, utilise?

[RokeJulianLockhart]

I've suspicion that it's not LZMA2, because, very unscientifically, the files appear larger than I expect them to be.

I ask because, if not, I want to suggest that the user be able to choose the compression method. The reason that I care is that KDE's Bugzilla instance restricts attachments to 4 MiB, and I doubt that retroactively encapsulating the zip in any alternative format shall produce better results.