Do not under any circumstances attempt to refill the little gas canisters.
It's just too dangerous to attempt it.
Blends vary from brand to brand, so I will just use MSR IsoPro as an example.
IsoPro is a 20/80 mix of propane and isobutane.
If you attempt to duplicate this mix and get it wrong or use just straight propane it will blow up in your face. Propane has a much lower boiling temp than isobutane does, thus the reason for the thick heavy canisters for pure propane. The warmer it gets out, the higher the pressure inside a canister gets. This is also why it's a bad idea to heat your canister, wrap a windshield all the way around it or to pour boiling water on it.
If you use pure butane to refill them, your stove performance is going to suck when you get close to 30 F and if you do manage to get it lit, once you start cooking and the energy transfer does it's thing and it lowers the temp of the fuel canister, your stove will go out. This is the reason propane is mixed with it and isobutane is used instead of butane.
You may even occasionally see adapters on Ebay to refill them, don't do it. It is just way too dangerous over saving yourself a few dollars.
Yes the little 1lb propane cylinders are different, they are built to handle pure propane and as long as you don't overfill them, they do just fine. The little camping canisters are NOT built to withstand the pressure of pure propane.
The boiling point of propane is -43.6 degrees F, isobutane boils at 10.94 F and butane boils at 30.2 F. So you can see propane produces much higher pressures at the same temps as the other gases and below 30 butane is worthless as it wont vaporize so you can get it to burn.