The information you provide isn't enough in any case. What type of riding you do, what kind of mountain bike, more specifics about your body and riding position preferences? In any case all I can say is try smaller bikes if they feel too large for you. Are you fitting yourself to frames with particular seat and stem/bar setups?

You might want to check out stack & reach type sizing, gaining popularity these days, rather than focusing too much on seat tube (or even top tube) measurements.