It might be fairer to compare to a state of the art piston engine. Brief googling finds a DeNardis 40cc engine weighing 11 lbs and producing 7hp. Still impressive though.
There's a trick you can play with engines that only engineers really understand. A good engineer can fluidly trade economy, reliability, and mass. So if you want a couple ounce model airplane engine that outputs a couple HP and might run for ten hours before burning out at insane RPMs with awful fuel economy specs, those are widely available and fairly cheap. Or you can buy an inboard diesel that'll push a motorboat for insane cruising distances under hurricane conditions and maintained with a shoebox of cheap tools, but it'll be expensive and heavy, like 5 HP and it'll be 100+ pounds.
I have no question someone can build a 4 pound engine that outputs 3 HP or an 11 pound engine that outputs 7 HP. The only question is how long, and how much gas it'll burn. Something tells me neither of them would run my commuter car for 200+K miles OR get 30+ MPG, which is pretty easy for my extremely high tech, but heavier engine, commuter car.
For an even more extreme example, a Top Fuel dragster engine can make over 10000HP and weighs less than 500lb, but has a lifetime measured in seconds and fuel efficiency better measured in GPM instead of MPG.
(The Freedom Motors site looks convincing, until you compare it with their site from 11 years ago. That's Moller - Real Soon Now forever. Over 40 years of Real Soon Now on the flying car.)
[1] http://freedom-motors.com/ [2] https://web.archive.org/web/20050411075152/http://www.freedo...
tl;dr: affordable, efficient, light weight: Pick any two ?
And here's 3hp electric motor (550g), although that's probably an unfair comparison. http://www.hobbyking.com/hobbyking/store/__18177__Turnigy_Ae...