If you want a light weight street single, the upcoming KTM Duke 350 will weigh approximately 335 lbs. wet with a good battry, alternator, and 17" supermoto wheels. If you want a light weight / high spec small street bike, you have to pay for it - a 300 lb. Ninja 300R or CBR250R would cost $10k or more for the light weight components.
In comparison to dirt bikes, the frame, wheels, suspension, alternator, steel tank, bigger triple clamps, much different frame and swing arm (large cross-section aluminum vs. steel), much larger batteries, handle bars, mirrors, rear subframes, seats all contribute to the difference in weight between Enduro (super light racing dirt bikes) and non-racing street bikes. The VERY light Daytona 675R weighs 415 lbs. wet. A 600cc supersport track bike weighs about 370 lbs and costs $20k plus.
Also, we need to be very careful comparing published weights of older motorcycles with our new standards. The first year of the FZR-400 in Japan (1986) called the FZR-400 1WG had a claimed dry weight of 157 kg (345 lbs). This was when those weights didn't include battery or alternator, or engine oil, fork oil, etc. etc.
The 1989 version of the FZR-400 for the US market had a published dry weight of 172.8 kg (380 lbs.) which included the battery and the alternator. With a 4.8 gallong fuel tank, full, the bike weighed 410 lbs. wet with all other fluids.
The FZR-400 had an aluminum delta-box frame and had what we would now call super-sport level components. The new Ninja 300R doesn't. If it did, it would weigh less, and cost what a 600 cc supersport costs, whick is $10k - and no one (except maybe me) would buy it (see our thread on "Nobody will buy High spec 250-400cc sportbikes." http://www.advrider.com/forums/showthread.php?t=648218