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Yes the newer "muscle cars" have smaller more efficient engines, but they also have less weight to push around in most cases. New carbon fiber/plastic panels are replacing sheet metal and even some engine parts.

I think the biggest difference between pre-80's muscle and todays muscle is the way engines produce horse power. Today that new mustang may advertise 305hp but thats at a 5800 rpm and only 320ftlbs of torque at the wheels. How often do people run their car at 5800rpm?

Todays motors HP peaks at a very high RPM, where as, for example, a 1970 mustang's HP peaked at 3400 RPM with 370hp and 450ftlbs of torque at the rear wheels. Also todays muscle is geared more toward a high top end speed rather than a quick response. Due in part to rear end gears being higher for fuel efficient highway driving, and transmissions setup for fuel efficiency and highway use.

Gone are the days of the popular 4.11 rear end as we are seeing 2.23 gears as common choices. 6 speed manual transmissions have replaced the old 4 speeds, and turbo chargers are seen on common family cars. Not to mention those goofy 22" wheels that personally I have yet see one that looks right.