I have 'bat ears' & one ear is bigger than the other. Is it possible to make the larger ear smaller?

If one ear is protruding and big, it will appear smaller when it is pinned. A surgical reduction is then mostly unnecessary. With otoplasties today, you have the choice between the usual traditional methods and the minimally invasive stitch method.

Ear asymmetry is very common.   A traditional otoplasty "pins back" the ears.   An ear reduction can be utilized if one ear is significantly larger than the other one.  

 Good advice would necessitate viewing photographs or in-person examination. Generally speaking, otoplasty surgery will certainly be effective when it comes to making the ears less prominent;  sometimes this is all that is necessary for patients to feel good about the appearance of their ears. If, on the other hand, your ear length is of concern, then ear reduction surgery may be necessary. It will be important to consider the pros/cons associated with each option presented. Best wishes.