Weight is only one factor of the equation. My bike weighs about 560 lbs (255 kg), but is no problem to handle because the center of gravity (CG) is well-placed. My old bike was only 328 lbs (149 kg) but it was "harder" for me to handle because the CG was too high (IMO). The bike was too light and too "nervous" around curves - the additional 100kg of my GSX make it much more pleasant to ride, and gives me more confidence in the twisties. And the additional 90 horsepower don't hurt either.
But as Clint said, there are bikes of all shapes and sizes available. The Kawasaki ER-6N, for example, weighs only 318 lbs (173 kg) and has a 650 ccm engine putting out about 70 hp. A nice little bike ... today. Up until about 10 years ago, those specs would have made it a super-sport racing bike!
Today's "starter" bike is a 600/650 bike with 50+ horsepower. Back in the '70s and '80s, riders started out on a 2-stroke 125ccm that may have generated 15 horsepower, then moved up to a 250 or 400ccm bike. Back then, a Harley was a "monster" with it's 1200ccm Shovelhead engine. Today, Yamaha offers the MT-01 with an 1800ccm V-2 ... an no one even really takes notice.
