I'll try to only speak for myself
>What's the appeal? Why would you be into this?
There's myriad reasons, I'd say the biggest is the power dynamic and comfort that comes from the age difference; There's less of a social expectation for the boy to be confident/socially competent, and it's more acceptable or even expected for him to be sexually/romantically inexperienced or nervous. That makes for a very comfortable fantasy.
>Does it have some overlap with a gigantic women kink?
For me at least, yes, although usually I prefer "Mini-giantess", ~7-10ft tall. Some GFD/role reversal is very appealling because of the social freedom like I mentioned, and size difference definitely improves that. Andin a way, the larger a woman is relative to you, the larger her breasts/hips etc. are by extension, so there's that.
>Do you want to be a little boy? Why?
I don't want to be ME when I was a boy, I was pretty gross, rude and lonely. I do however find the idea of living out a (lewd) perfect fantasy childhood as portrayed in a lot of straight shota media(i.e. Miss Caretaker of Sunohara-sou) incredibly appealing and comforting.