* Monkeys and apes are both primates. There's a lot of overlap in their characteristics.
* Size varies greatly within both groups. You have tiny marmoset monkeys and massive gorillas, which are apes.
* Some monkeys are larger than some apes. For example, a male mandrill monkey can be larger than a female gibbon, which is an ape.
It's more accurate to say:
* Apes are generally larger and heavier than monkeys.
* There are exceptions to this general rule.
Let me know if you have any other questions about primates!