So why 175/65R15??

No. It's between 2-4mpg worse depending on OE tire options. Aftermarket non-LRR tires will drop mpg even further.

The only thing I can relate about tire differences on a Prius...

Gen II OEM 185/65 15 Goodyear Integrity had spongy sidewalls, even at higher PSI. The tire was a poor performer (scary) in rain. On the Interstate, this tire added to the wandering nature of the car.

I had researched the European version of the Prius and noted they used a 16 inch OEM wheel. So I decided to try that size to see if it would help with driving performance.

I changed over to a Goodyear "TripleTred" tire on a 16 inch wheel, and the car changed dramatically. It was more planted and didn't wander at 65 on the interstate. Turn in was sharper and those all important MPG figures didn't change. The tire diameter was only a tiny bit bigger, but the GPS showed 53 MPH at an indicated 55 on the speedo.

Rain performance was excellent with this tire. Kept pressures at 40/38 front/rear. Ride was still comfortable.

Same sized tires from many brands can provide uniquely different performance feel. There are LLRs that are more LLR than others. There are tires that haven't been rated LLR and can still provide equal MPGs. It's all due to sidewall, and tread area carcass construction and rubber compounds that are used. A 195/55 16 tire from Goodyear could be significantly different compared side by side with a 195/55 16 Michelin. There will be different ways that the tires spread out under load and even how the tire shoulders are designed. Tread width and sidewall measurements won't quite line up equally from one tire brand to another, although they can be listed the as being the same size.

Grip results for any given tire on a car are usually a function of internal tire structure, rubber compound, tread design and contact patch size at given temperature and on the same piece of asphalt. There will be an optimum range in tire size for any given car, but you can still under tire and over tire a car.

I think I'm over-tired after all that.