Electrical tape or similar is great for making a makeshift splint, takes up very little room.
Also, instead of gauze pads, you can buy sterilized cheese cloth for little to nothing and vacuum bag it in various sized bunches.
Travel size pump spray containers can be had cheap, this allows you to carry small quantities of alcohol, hydrogen peroxide or antiseptic in small sealed bottles to prevent leakage in your kit.
Mouthwash can double as an antiseptic in a pinch, burns like hell but will work.
Look at Slice, they produce a line of ceramic scissors, knives and scalpels that are very reasonably priced and compact.
Also, most airlines have very extensive first aid kits and they are required to replace them on intervals. I went to a safety seminar where we were offered these kits for nothing since they can no longer use them. Worth a look at your local hub or contact the airlines directly about attending one of these emergency safety seminars and see about picking up an "expired" kit. The one I received came in a nice resealable dry bag backpack. I just replaced the missing expired med packs with fresh vacuum bagged OTC med packs and added items that were needed, like a snake bite kit.