two fast questions!!!

[BrianG]

It could be the BEC. Anything over 2s or 6 cells is going to cause a lot of heat, especially with a larger servo and/or heavy truck (servo works harder to turn).

However, the BEC circuit is on the underside of the ESC (near the USB port), so the heatsink isn't doing anything at all for it. If the heatsink is hot, the vast majority of the heat is coming from the FETs and the issue is with motor selection or gearing.