With my pony it is so easy to keep your lower leg still but with my new horse it just constantly is moving slightly all the time. He is quite fat at the minute cause he was turned away, could that be anything to do with it?

l think it is just an adjustment to your new horse. Give it some time. You will adapt to his shape and he will get more fit and his gaits may even out some, too.

Try some lower leg exercises like riding without stirrups, posting trot without stirrups, three seats (sitting, posting, two point for 5 or 10 strides each), posting one beat down and two up (down up up down up up), and lots of two point.

you just need to strengthen your lower leg and it will improve

If your new horse is quite a bit wider than your pony it can definitely make it harder to keep a strong, still leg on him. Since your legs are spread farther apart when you ride him you will be using different muscles than you're used to and it will take time and work to condition those muscles so that you have as still a leg as you do on your narrower pony. Take Greg is advice for strengthening exercises and in the end you will be a better, stronger rider overall.

l ride about five horses a day and each time l rode one for the first time l would have different problems. One of them made me feel like my stirrups were too short, even though they felt perfect on another. One made me sit different. And one made me feel like my legs were too loose, too. It just takes some getting used to. All horses are different and require different muscles.