First off, the game won't increase the number of players, as in the random debut code, it determined that it only needed to grab 44 players per team (basically, random debut is not set up to run with a minor league system, so it only needs an MLB squad plus enough of a taxi squad to come in for injuries).
And then once teams have drafted the 44 players per team, some younger players import essentially on minor league deals, so when the AI initially sets up the roster, they don't bother promoting them right away. However, when you advance to the season, they will call up the better ones to fill out their MLB roster.
They'll fill them out when the season starts. Basically, the AI is just being smart in not starting people's time on the secondary roster before they need to
