Cell's concept art in the title page explaining all of Gero's creations says he is imbued with the genetics of multiple types of creatures, not just those 5 people
Gohan said he'd only been meditating like that for the past several days. Before that he presumably trained SSJ Gohan harshly, since when he first accessed SSJ Gohan couldn't even maintain it for 10 seconds.
Generally training boosts start bigger and get smaller as time goes on. By the end of the ~year he spent in the RoSaT Goku decided more training wouldn't increase his power at all, and switched it up to relaxing his body. It's the same irl with people who just start out lifting weights or...
That seems odd since controlling his three Berserk forms seems the priority for Broly... unless he did that offscreen like how Galu and Vegeta achieved Blue in the RoF movie :troll
Does Piccolo take this big form after Cell shows off his magnificent redness in his Cell Max incarnation? Copycat...