Dogs pull for various reasons. But the simplest explanation is this: we keep following them, allowing them to lead us around by the leash. If we keep following, the immediate connection is this - them pulling on the leash equals person moving. Pulling on the leash is irrelevant to the size of the leash, so you get no brownie points for a dog that is pulling by your side. A dog will use the pressure on the collar to signal when you should move. You are, in essence, training them to pull you around.
One of the steps for getting a dog to stop is to teach him that pulling you will immediately make you stop walking. Patience is your friend in this. The walk will continue when there is no pressure felt on the leash or collar.
