Learn Portuguese (Portugal): O gato dele é mais pesado do que o cão.

In Portuguese, you don't add an ending like "-er" to make an adjective stronger. Instead, you put mais (more) before the adjective. So, "heavier" is built exactly like "more heavy": mais pesado.

What does do que mean here, and can I just use que?

do que means "than" when comparing two things. You will often see just que used for "than" (so mais pesado que o cão is also correct), but adding the do makes do que, which is extremely common in Portugal and sounds very natural.

Why does the sentence say O gato dele instead of using a word like "his"?

dele literally means "of him" (it is a mix of de and ele). While Portuguese does have a single word for "his" (seu), it is also used for "your" and "hers", which can be confusing! Putting dele after the noun is the clearest and most common way to say "his" in Portugal.

Why do we need the o before gato dele?

In Portuguese, we almost always use a definite article (like o for "the") before a noun, even when it belongs to someone. So instead of just saying "Cat of him", the structure is literally "The cat of him": O gato dele.