I have been pounding on my knees for 20 years riding bmx freestyle and skateboarding.
I have used alot of different knee pads.
From what I gathered from your post you are looking for a breathable, small knee pad for minor use/abuse. (your not doing crazy downhill or stunts)
I would suggest going to the sporting good store and look at various other sports pads. volleyball etc.
One pad that we used to use for light use was simply neoprene(sp?). the same stuff that wet suits are made out of and some shin pads. (and knee pads if you can find them)
they are light and small but most don't breath well, depending on how they are contructed.
I currently use a pad made by K2. it pulls on like a sock, offers a good amount of protection, is small enough to wear under my not so baggy jeans and offers good protection. but they don't breath too well.......
you can also go the route of putting the pads on your bike. if you hit your stem, get a stem pad etc..