I'd bet my money that you can roll your own iCloud-esque Photo Library and it still won't be near the 100MB territory if you just use native libraries for the UI. Everything you'd have to do from scratch: cloud auth, encryption, syncing, compression, transcoding, etc. is all at most a few MBs combined, not to mention that all of these except cloud auth are already in the built-in libraries on iOS that any app can freely use.

What the apps here are doing is shipping whole (UI) frameworks and tons of old code and assets that aren't really used anymore but nobody dares to remove.