I have a popular hunter jumper riding helmet, fit for my head but it looks and feels like its to big. is it?

I had my helmet fit for my head by the owner of a tack shop. I love the helmet and the style but when i put it on no matter how much i tighten the straps i am able to lift it off my head about 1/3 of an inch and it DOESNT fell snug. also looks big for my head.... can anyone give me some insight or advice... help?