Translation:The moose is a large animal and the mouse a small one.
Why can we not use A Moose and a mouse, where is the rule here for specifying "The" which are we talking about?
Without any context surely A is as acceptable as The?
That is not a compound word, but this works in a similar way.
One can probably say that "susi on suurpeto ja minkki on pien(i)". Sounds pretty weird, or you have to repeat the word "peto" which also sounds weird. In Finnish, the order goes "susi on suur- ja minkki pienpeto".
Or if you don't want use compounds: "susi on suuri ja minkki pieni petoeläin".