THe collar is fitted tight, and it is a narrow one. He is just VERY strong. I went out and bought a gentle leader easy walk harness......I hope to high heaven it helps. I have a double loop leash that has a loop low by the d-ring, and he still dragged me to the ground. Believe me, I'm not exaggerating! I have done the "no pull" etc, but the squirells are too enticing, and believe it or not he is cat tolerant of our cat! ha ha
To testify that it does indeed work, all of my dogs were taught not to pull using this technique. Eventually I was able to do this...
The picture is a few years old, but that's about 125lbs of human and 270 lbs of dog. On rollerblades.
This is getting long-winded so I'll wrap this up, but please remember that no tool or technique you use is worth anything unless you can use it correctly; the timing has to be precise or the dog won't learn anything.