Oh my, that sounds like it would've been a very interesting story

The general idea may be worth revisiting sometime in the future. I mean, the question of what might actually happen if a human with the ability to either create giant monsters ex nihilo or else turn existing living beings into kaiju did end up in the world of magical technicolor ponies is one that could easily have multiple interesting answers, and none of them need to involve Celestia screwing said human over on purpose (which would be a bit out of character for her anyway, I think).

The size question could answer itself if we’re willing to simply accept kaiju creation as a blatant cheat power that just so also happens to conveniently include the ability to return to something like “normal size” -- not only would that mean that the creations/recipients wouldn’t be stuck in full hulked-out mode 24/7 whether they wanted to or not (making them among other things way easier to feed and house), but a giant monster that can just “disappear” again and sneak away is arguably scarier to anything that wants to be its enemy than one that’s just too big to hide and so can easily be tracked wherever it goes.