Talking Moose is right but there are other factors.
1. Many perch lakes suffer at least partial winterkills every so many years and the biggest fish in the lake are most susceptible so they will die out regardless of being fished out. The USA has shorter winters which usually mean less chance of winterkill allowing fish to die of old age rather than low dissolved oxygen levels.
2. The biologists in Alberta have prioritized Walleye catch rates over all else to the point of complete domination of walleye. They consume the forage base necessary to grow big perch and then they eat all the small perch and then you have no perch to grow up. They haven't realized this apparently yet. To my utter dismay.
3. Growing season - Fish are like trees. If you have ever cut a tree down you can count the age based on the rings. The reason you see rings is during the summer the tree grows fast and during the winter is slows right down. Fish are the same. So in the USA they have a far longer growing season. Check out this great video on aging fish by Gord Pyzer.
https://www.youtube.com/watch?v=HaL7fXZTmIw
4. Growth Rate vs Age: So given #3 the fish are far bigger at a younger age in warmer climates. Meaning our 12 inch fish may be 2-4 years older than the same 12 inch fish south of the boarder. Then throw probability in there and it is much easier to get a big fish based off fast growth rates than avoiding predation and fishermen for a longer period of time.
Despite fish in northern areas living for a longer period of time to achieve their size, the growth rates are substantially less and the older a fish gets the slower its growth rate gets. Therefore you get really old fish that never achieve the size that the fast growing younger fish get to in the south. Even if the USA fish die faster.
5. Genetics and Forage base - Take for example the European Perch. Different genetics and different forage. Different more aggressive behavior eating larger food items equals perch measured in Kilograms. It's insane. The same so with fish across the USA and canada. Genetics varies and some just have a predisposition to reach larger sizes. Now that being said the current Jumbo Perch Producer in the USA, Cascade lake, was stocked with parent perch from a stunted population of perch. Therefore genetic potential was there in the fish, just masked by overpopulation and competition.
6. Those small ponds often are crawling with food. They are warm, tons of nutrients and the whole depth of the pond is shallow which means that every square foot of the lake is food producing. A lot of our lake don't have much food production besides the shallower edges.
Hope that wasn't too long winded. I find this stuff really interesting and even though I'm no fish biologist these are the reasons I've come up with.