"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

[WilliamHou15]

Why not Lwy to wielkie koty

[Jellei]

Normally I'd say that "wielkie" is more than just "big" and may be closer to "huge", but in this particular example, "wielkie koty" is a term used in biology and its English equivalent is indeed "big cats". Our sentence is more like a simple and perhaps a bit childish description. Anyway, added "wielkie koty".