In my account I have
"Ultima IV" *and*
"Ultima 4:Quest of the Avatar" listed.

The first has 16 MB listed for GOG downloader and the latter 17 MB - but the contents (e.g. Richard Garriott interview) of both seem identical.

Any idea what the difference is or if it’s just the same game listed twice?

One is a freebie game and the other you bought in Ultima 4 5 6 Pack.

Which one is which?
And is there a difference as the 16 MB <> 17 MB size seems to indicate?

"Ultima 4:Quest of the Avatar" is the freebie, the other was in the pack. Dont know difference as I never installed the freebie game, because I instabought the collection as it came out.