When I'm morphed as any identity and use Pehkui to change my size, some things (like the eye level and hitbox) don't change, while other things (like reach and the player's appearance) do.
I'm not sure if this is a Pehkui issue or an Identity issue, so feel free to let me know if I should move to Pehkui to report this.

Minecraft version: 1.17.1
Pehkui version: 2.2.1
Identity version: 1.14.2-beta
Other mods: This happens both with and without other mods.

(It doesn't cause any errors or warnings in the console, so I can't give you a useful game log, sorry.)