In a statement given to IGN, Activision was clarified what will be contained on retail copies of the upcoming Spyro Reignited Trilogy. Alongside the entirety of the first game, portions of Ripto's Rage and Year of the Dragon will be contained on the disc. Users will still need to download the remaining levels from both sequels, but it should cut down on the size of the download required.
"The trilogy will ship with content from each of the three games, including the entire first game and a subset of levels from games 2 and 3. In order for the full Reignited Trilogy offerings, a day one update is required," Activision told IGN. Activision didn't specify which levels would be playable, so it could be the final one for all we know.
This means that if you can't immediately connect to the internet to download the remaining content, you'll still be able to play some of Ripto's Rage or Year of the Dragon. I don't think that matters much, though, as a physical version of the game typically contains everything needed to play the game. People who are holding out for a disc probably don't have the best internet or just want to have their game for a time when servers aren't available.
Whatever the reason for this decision, at least the download shouldn't be stupidly large.