That there is your problem.
That is basically a starvation diet. It is keeping the snake alive, but it isn't growing enough.
It is completely normal for the body (middle) to be wider then the head, neck and tail.
A rat pinky is something you would give a hatchling fresh out of the egg. By now your snake should be eating rat weanlings. You have to go up in size of food. Slowly of course..
Imagine the rat would stretch out to reach something...the width of its body should match the width of the Ball Pythons WIDEST part, the middle part. Start feeding appropriate sizes. Since your baby is somewhat stunted, go slow, though. Give it a rat fuzzy for the next 2 feedings and then go to rat pups. From then on, match the middle part of the rodents and snakes body.
Also weigh your snake, so you can keep an eye on the weight gain and it also helps in determining which size to feed. A cheap digital postal scale works fine for that.
Basically, your snake hasn't grown enough to shed its skin..
He/she will catch up though.