Has it been? I thought I've noticed the sizes getting a bit smaller the last few years, thanks to developers getting better at compressing the files. For instance Capcom just announced Res 3 Remake + the Resistance multiplayer mode will only be about 43GB on Xbox One, with Res 3 Remake only taking up 22 GB if you choose not to install the multiplayer.
Checking Xbox Store right now, I see FIFA 20 at 42 GB, Madden 20 at 43 GB, NFS Heat at 28 GB, Sekiro at 12 GB, Plants vs Zombies BFN at 24 GB, Jedi Fallen Order at 48 GB, Far Cry 5 at 46 GB, The Outer Worlds at 36 GB, Greedfall at 12 GB, Kingdom Hearts 3 at 37 GB, Mortal Kombat 11 at 48 GB.
The main problem seems to be some of the open world games. Some, like Witcher 3 aren't huge, it comes in at a surprisingly small 40 GB, but AC Odyssey with DLC is 140 GB, RDR2 is 101 GB. Many others falling 60-70 GB range. There's also CoD MW at a ridiculous 150 GB, and that's not an open world game, I guess Acti hasn't heard of compression (it's double the file size of DICE's Battlefield V).
Since I've just bought the gold edition of AC Odyssey, I can tell you that the base game is 66GB, the DLCs are 30GB and the AC3 remaster (that also includes AC3 Liberation) is 45GB (=~140GB in total). I don't think the sizes for Odyssey and the remaster should be combined for this comparison.
Also RDR2 is currently 119GB.
But I agree with you. The average file size is in the 30s to 40s.