While I do appreciate your suggestion (Stage based inflation can be debated and I do agree with that), the current size of Xap is already doing enough than an already bigger size.
Making Xap any bigger than that would make the stage incompatible with the extended size, leaving the final stage redundant (Ex: Much bigger target, corridors are too narrow to dodge the enemy’s attacks, getting stuck on walls, to name a few).
However, I can see this idea only applicable to a very select areas on specific segments like one of those Wario Land Transformations (Ex: A vertical scrolling segment where you inflate to the biggest size, but then you’ll float up and dodge obstacles and won’t deflate until you reach the ceiling). You unfortunately can’t have access to the last stage since you’ll need an item to inflate to the biggest size.