"The mouse is bigger than the cat!"
Translation:Die Maus ist größer als die Katze!
In this case, 'größer' stands alone and is neither affected by the two feminine nouns. It simply means 'bigger'. 'Größere' would have been used if it were qualifying the mouse, as in 'die größere Maus' i.e 'the bigger mouse'. Hope I've been of help. I'm learning as well, so pardon me if my explanation doesn't satisfy your need