Well, you're running both the iOS development tools (Xcode, iOS Simulator), plus the Android development tools (Gradle, Android emulator, and maybe Android Studio too). These add up.

16GB might be possible, though.

(Skip itself doesn't take much memory. If you run it headlessly as a SwiftPM plugin, you wouldn't need nearly that much.)

IDEs targeting mobile development saw the bloat in FPGA IDEs and said "we can beat that".

I assume that’s for the development phase, not (necessarily) for apps developed with Skip

Likely because it uses both iOS and Android toolchains plus its own transpiler (with Skip Lite) or other overhead with Skip Fuse. iOS alone is already challenging with 16GB. Don't blame Skip for this - it's on Apple and Google for not shipping memory-efficient tooling, which shouldn't be a surprise if you've used their software.