I think poor packing or packaging has something to do with dudes carrying fat briefcases. I find that if you go beyond 17 inches in width or 13 inches in height you will rapidly start to lose foot room underneath the seat in front. (one of mine, for example, is 16 by 12 by 6) Also great looking leather cases with nice thick leather takes away packing room. Great for the office but tight squeeze when you travel.

BTW. this doesn't apply to IT people carrying around a bunch of electronics. Since I only carry a laptop, power cord, and a bunch of smaller peripherals, I wouldn't know how to comment on those things.