The separate amp likely has a larger power supply and bigger heatsinks to keep it running (relatively) cool and that is why they are bigger and heavier. Separate amps are usually designed to drive all channels simultaneously to their rated power while receivers are usually rated to drive any 2 channels simultaneously to their rated power. When a receiver's specs say 7 X 170, it means that there are 7 amplifiers and each is capable of 170 watts - NOT that it can necessarily drive all of those amplifiers to their rated power at the same time.
Search for the 'All Channels Driven Controversey' thread for more info. The short of it is that real life program material will never drive all channels simultaneously and receivers/amps rated two channels continuously may have better 'dynamic' power as it can apportion the power needs as the music signal changes. Either way 170 watts per channel is way more power than you will ever need unless you have a very large listening room and like to listen at ear bleeding levels.
