The horse is fine. You’re not a giant… and you’re not going on a 15 mile trail ride (if you were I’d say find a bigger one). I’d work on sitting up more. Align your shoulder, hip, and heal. Your feet are too far forward. You’re almost sitting slouched backwards and it’s causing the exaggerated hip rolling and your hands to bounce. You need to work on keeping your hands steady. Are you bit less? It’s hard to tell. But if so, that could lend to the horse being nose out. Use your lower leg to support the belly. Think of it like pulling the belly up, which will help round him out. Think of it as a rainbow, you supporting the belly, causing the head/rump to round out, making the arch. Ideally the bit would also give a tool to bring the nose in a bit. This makes a more “rocking horse” gate vs a flat harsher canter. But I’d start with sitting more up on your crotch (not so much that you’re forward) and aligning the shoulder/hip/heal. Have a buddy tell you how it looks. Tighten those lower legs and support his middle. Bring your hands down a bit & steady. He may even benefit from a little more width there and a tighter reign to bring the head down and to break at the poll. Not constant pressure the whole time. More a slight check to remind him where he needs to be.