"A wolf is a big dog."
Translation:Wilk to duży pies.
that does not work like that. In this sentence "to", "to jest" or "jest to" mean exactly the same. (although "jest to", feels like there should be some longer description. "Jest" means exactly the same but needs different case
Polish does not have difference between "a big dog" , "big dog" and "the big dog"
The problem with "wielki" is that it seems bigger than "big" and smaller than "huge", but we more often translate it as "huge". We consider accepting this word on a sentence-by-sentence basis. As I can draw some analogy to "big cats" (wielkie koty), I guess this can be accepted here. Added.