Actually, it’s not. Our house was built in the early 1980’s and it’s fairly well insulated. It’s not that well sealed though, which can cause issues on windy days both during heating and cooling seasons. Here in SW Ontario it can get very humid in the summer. We keep the house at 75f during the summer months and find it very comfortable. If it got much hotter here in the summer (it’s coming) I would want a larger system. Going with a smaller system versus larger allows for better dehumidifying, and given my choice between drier and warmer versus cooler and damp, I’ll take dry. A larger system would cycle on and off more frequently and leave the house more humid.
I suppose that a 400-600 sq ft uninsulated building may require a ton, and as you say depending on where you live. A 4 ton central a/c unit needs to be on a 100A (50A tandem) breaker to allow for start-up current and should run at around 20-25A @ 230V.
We had a house a few years back that had an oversized a/c unit (1800 sq ft bungalow with 2-1/2 tons) and it was horribly uncomfortable. Felt like we were living a cave. I had to pull every trick in the book to get that a/c unit to do a decent job of dehumidifying to keep the place comfortable.