This means two things;
They will have high compression as well as more displacement.
With the same piston crown shape, they will have a higher compression ratio than the smaller pistons.
Big bore kits are actually hi(er)-compression/big-bore kits.
All of the 944 kits I've installed have had the advantage of porting, so I can't show dyno runs of stock ports vs stock ports. I can show dyno runs which (in my opinion) are somewhat representative of the differences.
The chart below compares two bikes with porting. One with 94mm and the other with 92mm pistons.
The notes for the 904cc bike say it has bored carbs, but they are straight bored and offered no increase in performance.
The third bike is an unported high-compression (11:1) bike with none of the other little fixes I've learned in the past year. I'm a little disappointed in the output.