In EMU's 1 power car is required for 3 coaches so for an EMU 12 coaches in length will have 4 power cars.
Whereas for MEMU's 1 power car is needed for 4 coaches.
So i am guessing MEMU's generates more power (OR they are more powerful) than EMU's. Thats why MEMU's are normally 16-20 coaches long whereas EMU's are generally of 12 coaches only.