BMW is a producer of mass market cars (the M-coupe may be a special case) They have to engineer a car that can pass safety regulations around the world. To do this they must add structural supports, the so called "safety cage" around the occupant area. This adds weight (as does government mandated door beams, air bags, and customer demanded stereos with CD, power windows, leather seats, etc...) They could save weight by using exotic materials, as they have done in the Z8, but who would pay a $5,000 more for an M-coupe in order to save 100 lbs? Its a balance act the engineers must follow. Low volume car makers like Porsche and such are less concerned with cost, and more concerned with performance-- as they should be. In the end, weight has little to do with anything, if the engine, brakes, and suspension can handle it. As a general rule, BMW cars have shown that they can handle thier weight rather well.