With much higher compression you’d have to redesign the engine with more strength and weight, thus a negative impact on real mileage. This really becomes a factor on diesels, with far higher compression, but can’t be ignored even in the 12:1 range, IMO
Some engines would need to be redesigned, yes, but they wouldn’t be as heavy as you’d think.
Part of the reason why diesels are so heavy is that most diesels are designed to take 19.5 to 20:1 compression ratios, as well as much higher power densities. Unlike a gasoline engine, many diesel engines are designed to cover a range of HP outputs. The same engine block in my JD 4440 tractor (a 466-cu in inline-6) at 135 PTO HP is in the 4840 tractor at the high end of the line, and that engine is regularly “turned up” to produce 200 PTO HP. Just push in more air with a bigger turbo, turn up the fuel delivery at the injection pump and you increase the HP. Deere designed that engine to withstand the demands of the high end of the power output, then just put on smaller injectors, pumps, radiators, etc as they went down the power scale.
Today, diesels in trucks, tractors, etc - are commonly controlled by a computer. Change the firmware on the computer controlling the fuel injection pump and you change the output. The engine is still designed for the high end of the HP output range. That’s why so many diesels seem so massively over-built.
So designing a gas engine to withstand 12:1 compression ratios isn’t going to result in a block that looks like a Cummins 855 stuffed under your Geo’s hood. More than likely, you could accomplish a compression ratio increase to 11 or 11.5:1 on many engines by changing the pistons and heads and leaving the block in place.