Great questions! By "pet store goldfish" you mean vs wild, pond, import or breeder? Yes, they can get a lot smaller than Thai goldfish (some breeds get upwards of 16") but generally, most end up around the same size. I'll explain:
Goldfish size depends on environment, breed, care, genetics, temp & diet. They typically grow bigger in large tanks, high temps and given high protein. Pet store goldfish are usually stunted but can grow out of it. Here are typical sizes for each breed:
Bubble-eye, lionhead, celestial, telescope, pompom, froghead, shukin (4-8"), ranchu, veiltail/fantail, jikin, izumo nankin, pearlscale, ryukin, short-body oranda eggfish, tosakin, imperial, tamasaba, sabao (6-10"), shubunkin, comet, hibuna, wakin (10-16"), koimets get up to 24". (10-20cm, 15-25cm, 25-40cm, 61cm)
Hope this helps