Going through a “squishy” stage is much riskier for a large terrestrial animal than a small one! Evolution has produced amazing adaptations — but it cannot sidestep the laws of physics. And the laws of physics make it clear that a large terrestrial animal with no means of protection or body support will deform, have a difficult time moving, and may end up as somebody’s lunch. Natural selection would probably act against a large animal that faced so many extra challenges when molting.