Being thinner gives you more surface area though (for the same volume). Isn't this technically better for heat radiation?
edit: rewritten to be more clear.
A heatsink isn't a simple box, it's a stack of fins. Adding up the surface area of a stack of fins is almost exactly how you learn to calculate volume at the start of calculus. Heatsink surface area scales linearly with volume, not external area.
Secondly, case thickness is not 1:1 with heatsink size. The heatsink has to share that space with other things: Case panels, motherboard, cpu (or gpu die), mounting hardware, required clearances under the pcb, etc.
But all of those numbers are fixed, they don't change when you make the case thicker or thinner. So the smaller you make the case, the larger percentage of room they take up.
So if all the extra stuff takes up 25mm, and your case is 100m thick, your heatsink can use 75mm (75%) of the thickness, but if your case is 50mm thick, you only have 25mm (50%) of the room for it. So reducing your case thickness by 2x reduced your max heatsink size by 3x!
(Note: these are made up numbers and probably exaggerate the issue.)
So it's actually the reverse of your claim, thermals get worse at a greater than 1:1 ratio as you shrink the case thickness.