bhaari to mein hun - kya karein... I think its called being "overweight"
anyway
engine has specific output at given rpms (measurements)
its like a generator - you apply load (lights, reefer, freezer, room fans to it) - the genny engine labours as its works - its rpm is controlled by its governor.
similarly you apply a load to the engine - it will actually be "measured" as reduced power to the drivetrain because some power is being sapped up by the A/C compressor as normal work.
whats so hard to understand - or does someone want some weird "susst aur bore lecture type" mathematical and physics calculations?