Bison

@V think t shirt sizes. So L is 'sized up' from M, S is 'sized up' from XS, etc

@lioncitysolver every theme answer has one or two squares that are a T-shirt size that have been sized up meaning M becomes L etc. It's a little Yankee that there are two instances where two squares become one or one square becomes two because of the x for extra small or extra large.

@cbdub Thank you I can't believe they messed that up!