Gorillas are big because they are leaf-eaters

Goril­las are big and strong. Like most of the big and strong land ani­mals, they eat leaves. Ani­mals that main­ly eat leaves are called foli­vores. (Foliage means leaves.) In this arti­cle (http://www.jstor.org/pss/4219431), some biol­o­gists explain why you would expect a goril­la to be a leaf-eater:

Large mam­malian her­bi­vores, accord­ing to accept­ed eco­log­i­cal the­o­ry, may be expect­ed to feed on abun­dant, low qual­i­ty food as a result of the rela­tion­ship between their body size, meta­bol­ic require­ments, and gut capac­i­ty. … Abun­dant low qual­i­ty food means non­re­pro­duc­tive plant parts, i.e., stems and leaves, because fruit and flow­ers are too ephemer­al to pro­vide a reg­u­lar source of food in bulk for large ani­mals. So, large her­bi­vores ought to be foliv­o­rous in a broad sense, and usu­al­ly are.

Real­ly big ani­mals need to eat a lot of food. Flow­ers and fruit may be tasty but are in short sup­ply. For this rea­son, the real­ly big plant-eaters tend to eat most­ly leaves.