I found the response I've been looking for. Or at least a response that does what I have been looking for a response to do—a response for when people squeeze you meaningfully after having already spent several hours with you and having already told you that you look nice and say, "You look great."
I have been looking for a response that accepts their kind intentions but gives them some sense of awareness that I take issue with some other part of the message there—the part that congratulates you on being less fat, with all the assumptions about your intentions and feelings that it carries.
It turns out it is really simple. It popped out of my mouth today in just this situation. It goes like this:
Here's why I think it works when it's a situation that doesn't actually set me on edge: most importantly of all, it maintains cordiality, it keeps the interaction friendly and affectionate. And it also reëstablishes parity and equivalence: my body is like your body, it's just a body, it has unity and integrity no matter its size relative to other bodies or other iterations of itself. And it also carries—if the person to whom you offer it in exchange has not him- or herself recently become slightly less fat—a mild rejection of that comment's subtext, because it chooses to take "you look great" at face value, to make it a compliment that can be given back to someone who hasn't recently changed size.
I like it.