Translation:Elephants are large and cats are little.
The definite article 'La/Le/Les' can be used to state general facts about a noun which follows it (J'aime la bière - I like beer [not a specific one but the drink in general]). If you were to reasonably interpret the French sentence in question into English, you would have to omit the 'the's - because here you're not talking about specific elephants and cats but about those two in general.
"Les éléphants" and "les chats" can represent 2 things:
- specific elephants and cats = the elephants /the cats
- full categories of things, elephants and cats in general, all elephants and all cats = elephants and cats.
Both specific objects and generalizations need the definite article "les".
If only some elephants and some cats were described, the French would probably have "certains éléphants sont... et certains chats sont ...".