Technique determines the ability to hit a heavy ball. I once say an 8 year old girl will pig tails wolloping the heck out of the ball. She had a fantastic swing but any adult on these boards could beat her at arm wrestling.
Strength and fitness is very important to tennis. It helps you get to the ball, improves your stamina and helps avoid injury. But, Chris Evertt hit a very forcing 2 HBH and she had arms smaller than mine.