Mickle

This is an archaic term that suffers under the burden of meaning much (or many) in some instances, and small (or few) in others. The OED (229), Samuel Johnson (165), and then-contemporary dictionaries (77, 250, 290) all focus on the former meaning alone. Nevertheless, in current usage the second meaning seems more likely to apply. That is presumably Gilbert's intent here (48, 294). See The Yeomen of the Guard for another example of the same interpretation.

Context: 
But his gallantries were mickle
Opera: 
Act: 
Act II