@Jud Crandall hit the nail on the head. You have to learn to dial it back. I know exactly what you are talking about, though. I am capable of making myself out of breath on 1 rep deadlift.
In one of Pavels books, I forget which one, he makes an analogy of a stereo system to our bodies and strength training. I think it goes like this: The stereo device is our brain and CNS, the speakers are our muscles, and the volume knob is our intra-abdominal pressure.
If your max squat was 400 lb and you were squatting 100 lb, nothing should change with this lower weight. Not your will power, tecnique etc... However, you do not require the "volume knob" to be at 100%, so you can lower your intra abdominal pressure. If you were only doing 1 rep at this light weight, you may keep this intra abdominal pressure high. But if you did 5 sets of 20 reps, you would be forced to lower the "volume knob" or else you'd be gasping for air in between each rep.
Start by practicing with just a few swings, maybe 5. Practice just pulling in 20% of the air or 20% of the intrabdominal pressure. Experiment with different perceived %s. Unless you are just going easy (like a jog), the set should be anaerobic, not aerobic. Anaerobic meaning without oxygen. Therefore you should be breathing heavier after the set. You must replenish that oxygen from the anaerobic set.