it's gotta be something like mostly media (uncompressed bitmaps), metadata, (debug symbols maybe?), libraries / platform / compatibility layers (95% of which is never used for any particular install).

I have seen what it looks like when you actually try to cram "features mostly" into a few tens of megabytes and the GMail app is not that.

source: the demoscene

That's exactly what the Gmail app does too. What other features does it offer that the Mail app doesn't? If none, then why does it need to be so huge?

I picked on Gmail because it's the most recent culprit that I have noticed creeping (sometimes jumping) up in app size but it isn't the only one guilty of this. Like I said in my original comment, the Reddit app is 13MB and does a bit more than the Gmail app. I can read/write posts and comments and view images; similar features to Gmail, but I can also watch gifs and videos. Can someone explain why there is an order of magnitude of discrepancy between the two apps sizes?