Paul is 15 or so in the book, he’s meant to be a kid and in the first half of the book he is definitely a boy. There’s a small time jump of a couple of years in the novel, which I assume is where the cut-off point in this film will be placed in which he becomes more of a man. (He’s not ‘meant’ to be the KW either - Jessica was supposed to sire a girl who would be the mother of the KW, but because she fell in love with Leto she gave him the son he wanted and subsequently the BG aren’t sure what he really is)
If this does well and they start planning for the 2nd half of the book to be filmed, there’s a decent amount of time for him to fill out his frame a bit, although really his power comes from his combination of Other Memory, presence and idolisation by the Fremen due to their religious engineering by the BG, not from big muscles.