good contact permits good energy transfer and heat absorbtion

when your brushes skip it arcs and when it arcs it generates lots of heat and sometimes burns the coppper, on top of that, brushes act as a heatsink, ever wonder with on good mills end-bells (havent used that term forever) have little aluminum heat sinks on them.

so in the end, because its not a good match, your causing the motor to arc, scorching your copper, and is making it get very hot.

DO NOT use incorrect brushes on the motor, you must get proper brushes, its very important as its the reason to break in brushes- to properly mate the brush and commutator (another old word)

get the right brushes.