I'd guess a decent FWD system is probably as light, if not lighter than an RWD one and you don't have to pass the chain under, over, or through the rider. Chainring sizes vary somewhat as the use of some kind of crossover drive with step-up gearing means you don't necessarily have to have a really huge ring, but PulsaR, the Italian bike, has a 108T single ring and RWD. Anything up to about 65T is reasonably readily available but above that they tend to be custom jobs.