English quirks: when comparatives work in reverse

Look at the following autobiographical paragraph:

I'm a fat guy, and I've always had a barrel chest. Even when I was thinner—I've never been thin, mind you, but I've been thinner than I am today—the area between my solar plexus and my navel has always been convex but muscular, despite a distinct lack of exercise. Strange but true: this is my life as a barrel.

Note the three highlighted words: thinner, thin, and thinner again. If you think about it, the two instances of thinner have somewhat different senses. The second thinner functions the way you'd expect it to: as a standard comparative of thin ("Bob is thin; Burt is thinner"). It's the first thinner, however, that operates a bit strangely: if we compare the first thinner to the thin, we realize that thin is describing a skinny Kevin who never was, while thinner is actually describing a fatter Kevin who did exist.

Kevin now and since forever: fat
Kevin 20 years ago: thinner than now (say, 90 kg)
Kevin never: thin, i.e., actually skinny (say, 60 kg)

So: thinner designates Kevin at 90 kg while thin designates Kevin at 60 kg. Bizarre, no?

You could argue, I suppose, that there's nothing strange or self-contradictory happening here because the thinner is clearly referring to "Kevin when thinner than now." Still, what the first thinner represents is a Kevin who is heavier than the hypothetically thin Kevin (see above), and I find that quite bizarre.

There has to be a name for this sort of topsy-turviness when using comparatives a certain way. Does anyone know the term for this phenomenon? It's probably some Greek word—a -phrasis or an -esis of some kind.