But his groundstrokes including return are not weaker than Karlovic's for example!
Yes. Karlovic is all serve though, so is better the faster the court. Isner is more than serve, but he can't use the rest of his game on grass or faster hard.
Anderson is the same, he's said he prefers a midpace high bounce hard court. It's also true for some normal-sized guys like Tsonga, who does best in Australia (sf/qf/final last 3 years) cos he can actually make some returns there.