" I don't get why people are anti-shooting on bigger guys."
Bigger guys teach you how to shoot properly. With small guys, you can get away with an ugly shoot attempt.
Ideally, you are supposed to shoot in on someone when they are falling forward and taking weight off of the leg that you are shooting on (for a single, my favorite).
The worst thing for you to do is walk right up to a big guy and do a single while he is standing there waiting for it. Then when you hit his leg, it is unmovable because all of his weight is on it. Then he will grab hold of you and smash you with his weight.
The best thing to do is make them commit with their weight. Tie up with them, do an arm drag or a wizzer...and do a single leg from that.
Also, when you do a single leg, you are SUPPOSED to drop down, move forward, grab leg, THEN STAND UP QUICKLY. If you do it properly, then their leg will feel "weightless" because you are grabbing it while they are falling forward. Timing is VERY important.
This is very much like a beginner throwing low kicks. Most MMA fighters do not setup their low kicks. They just walk forward and throw. No setup.
Throwing low kicks and shooting in on someone BOTH require setups to do them properly.
If you use proper technique for the single leg, and you land the move, your opponent is going down. If someone lands a perfect single leg on Randy Couture, he is going down too (though he might get right back up 5 seconds later)