Some ponies are naturally a little on the chubbier side. You don't need to worry too much. But and like Alannah said, u can take some grains out of their diet.
Some horses don’t need a lot of grain. We have a horse that gets hay and only a small handful of grain (just for nutrients) and he is a good weight. Maybe if you have your horse on grain now then lower it and maybe if you need to you could end up taking it out of her diet completely. But many horses need the nutrients from grain, so it is a good idea to keep her on a little bit.
I would also check to see if she is insulin resistant.
You should put your pony on a diet and as well keep exercising him. You can also add more exercise. If you ride once, then try riding twice a day.
We had the same problem with a pony at my stable. We gave him half a flake of hay and a handful of grain and worked home everyday.