I noticed the same thing today. I just installed a heat pump for my swimming pool. It has an MCA of 35 amps and I used #8 copper wire on a 40A breaker. The 40A breaker gets warm after an hour or two of the heat pump running. I was also surprised to notice that the flexible metal conduit containing the wires was also warm (there's a separate FMC circuit run next to it, and it was quite cool). The 60A breaker feeding my pool panel was also warm, as was the 100A breaker that feeds the panel containing the 60A breaker.
I think that once you reach a certain level of current draw (probably at least half of the circuit's rated ampacity), and it goes on for a while, things get warm.
My heat pump has run for over 5 hours and not popped any breaker. It says that a 50A breaker is the max permitted, but the 40A seems to be holding just fine.