It's supposed to be really loose so that it does not interfere with normal rein use. If you are an adult on a medium to large horse rather than a tiny pony then it will not be too big.
When you say its too big, what do you mean? I use one and it does not come into play unless you get into trouble. I can move my hands up my horses next to jump without it interfering with his/my movement at all.
I'm 5'2" lightweight adult on 14.2 connie. Haven't tried jump position with it but when I sit on her & raise my hand up it comes right up to my shoulder. I reckon I'd be off before it came into action if she bucks like she can!
Meant to say I thought I'd bought a medium but it says M/L on it.
Oh yes that does sounds a bit big, I just looked at a photo and mine comes about 2/3rd of the way down the saddle flap on my cob. Did you buy it from Libby's? They might take it back. Otherwise you could try selling it on ebay, they go for a lot of money there and buy a new one maybe?
Can you not just take it back to where you bought it from? I'm assuming you didn't buy the small size (for riders under 5ft 4?). If you can't take it back as used etc then you will have to sell and buy another, you could try advertising it as a swop on preloved and the likes but you may struggle to find someone else needing to swop for your size.