They're pretty much comparing the "best of the best" with the "worst of the worst".
There's quite a few very good NA cosplay, and quite a few -damned horrible- JP cosplay I've seen, granted it's also highly dependant on where the con is even held =/ yeah, many people in the US have weight problems nowadays, which can make for some bad costumes, but that doesn't mean they're all automatically horrible and that the ones in Japan are automatically all good.
As already mentioned, yeah, a lot of the costumes over there are premade, ranging anywhere from 5000 yen to 40000 yen from what I've seen on some tailor sites. You're damn right there's gonna be a quality difference when person (a) spends $10 making some gimmick in their house and person (b) who may have spent upwards of $350 to get (or even just rent) an outfit for two or three days.