Word also going around is that the original Spyro assets either don't exist anymore or Activision was unable to acquire them from Insomniac, so Activision basically had to remake all 3 games from scratch. When you consider that modern games are typically 20-30 GB a pop (for reference, the 2016 remake of Ratchet & Clank was 28 GB), it's not that surprising for it to be 50 GB for 2 games.
That said, this strengthens my theory that these will be download codes, not a patch.
Oh, and if you think that install is huge, try looking up how large Doom or Battlefront 2's current install sizes are. They're both near 100 GB.