+1 on checking the stirrup length. It took me a while to get this sorted out for myself. Too short and you're going to feel a little tight, almost cramped up, and it's going to make it hard to balance. It's also going to make it difficult for you to get the good flowing movement in your hips and pelvis that you need.
Too long and you're going to wind up putting your weight into your toes, not your heels.
My trainer told me that for most people riding hunt seat, the correct stirrup length can be determined like this:
1. Put the fingertips of one hand up on the top of the stirrup leathers, right up where all those flaps join to the part of the saddle you sit on.
2. Straighten your arm out totally (keeping your fingertips right up there at the top of the stirrup leathers).
3. Slide the stirrup iron down and stick the bottom of it (where your boot goes) right up into your armpit. Not just on the edge of your armpit where your bra goes - but put it right into the pit itself.
4. Look at the measurement on the strap that makes the stirrup long enough to stretch from your fingertips to your armpit. This is going to be close to the right length for your stirrups. Start with that, and tinker from there.
I don't think this is a matter of how big the horse is - the mountain I ride lessons on is over 16 hands, and a big blocky quarterhorse. It's all to do with the length of *your* legs.
Give it a shot - when you're tacking up and doing the final adjustments to your cinch before you get on, use this above to check the stirrup length, and move them if they need to.
If you make them too long, it is not at all a big deal for your trainer to fix this while you are mounted up. All you have to do is make the horse stand, get your foot out of the stirrup and move your leg in front of the saddle - your trainer will be able to make adjustments without you having to dismount. (My lesson horse uses a big fat sheepskin pad under his saddle, and it compresses *every* time I get on him, so I adjust the girth, I get on the horse, and then my trainer has to fiddle the girth some more...while I'm on him - it's not a big deal, I promise. :))