Experienced painters will tell you that "air dry" means "shrinks". The solvent goes away (that's "drying"), there's less stuff there, and so it's smaller. Pile up a thick enough layer of air dry putty, and it'll noticeably shrink.
2-part putty that doesn't have any solvents to dry out of it won't shrink -- so if you're filling some really big divot, or making fillets, that's what to use.