Just wondering... at what age did you stop using the highchair?
DD, at 17 months, seems to have less tolerance for sitting in the highchair for the same length of time. She gets squirmy (claustrophobic?) sooner than she used to, and it looks like she's trying to squirm her way out. So then I take her out, and we sometimes finish our meal while she's standing up (well, standing, and walking about, "playing" with something, coming back for next bite, walking about, ...
) so it's not like she's squirming because she's done eating. Is she already outgrowing the highchair? As for fit, she's on the petite size and fits fine, lots of room to grow.