I don't think you can make an air-cooled engine too much bigger without it becoming a heat monster. My guess is we'll start to see liquid-cooled bikes before too long. I'm waiting on a liquid-cooled Indian 4.
HD engines are already heat monsters. Lots of threads on HD forums re: people literally burning their legs from the engine heat. I had to put heat deflectors on my 96ci and I read that the 103's are pretty toasty.
Harley's are weighing in at 800lbs and up; how much more weight can people enjoyably handle? My Super Glide was one of the lightest "big" twins at about 680 curb weight. That's a lotta weight to maneuver through gravel parking lots, etc. Bigger is better? Depends...
