Spinning 626mm blades on 6S is will pull quite a bit of current even at the lower 1500rpm to 1900rpm range. If geared for 1900rpm, flying for long periods at the lower 1500rpm may also run the ESC hotter since the Castle ESC's don't feature active free wheeling.
You may want to check your CC 120HV logs and determine the max temperature and how much current you are pulling before decreasing to a 80A CC ESC on 6S.