"Lions are big cats."

Translation: Lwy to duże koty.

[Xodgetts]

Lwy są duże koty? How come "to" is used here?

[immery]

to is used with or instead of jest/są in noun is noun sentences.

Lwy to (są) duże koty.

Using "to" makes your life simpler because after "to" we use nominative case (the one you find in dictionary and use as subjects in the sentence)

If you don't use "to", you have to use instrumental case:

Lwy są dużymi kotami.

[Xodgetts]

Ahh, thank you very much

[pysio1906]

Or Lwy są dużymi kotami

[Wippii]

I guess this is better than the "A wolf is a big dog" sentence. :P
It's clear that biology or taxonomy is not Duo's area of expertise.