Mi hermano es más gordo que mi padre. es porque mi hermano come

Mi hermano es más gordo que mi padre. Es porque mi hermano come más

This sentence stands for a comparison, but what comparison? the comparison of a father and a son, that is, who is the one who eats the most?  so, más is an adverb that establishes a comparison between two terms. In this way, my brother is fattest because he eats the most.