There are two diffrences between them, 1) 증가하다 is an intransitive verb meaning "~ 이/가 증가하다", on the other hand, 증량하다 is a transitive verb meaning "을/를 증량하다", it needs objects 2) 증량하다 is more specific than 증가하다 증(增 = to add or increase)량(量=quantity)하다 증가하다 = to increase 증량하다 = to add the quantity \^o^/

I looked them up in the dictionary. It seems to me they have no difference in the meaning. However, the first one is much commoner.