Original Pixracer gets hot

I have two Pixracers and I noticed that they run very hot (> 80 °C). I cannot touch the ARM and even with a copper heat sink they run hotter than a RPi3 under load and without sink. I saw a guy in the PX4 Forum also complaining about this. Is this expected behavior? Why is a 180 Mhz chip that hot?
After some time on USB the heat leads to a disconnect.