If you are smaller, they there big guys are probably less able to defend the perimeter. If they are quick, they are more likely to gamble and overplay passing lanes opening up back door cuts and back screens. While very athletic teams have many advantages, most teams are equal enough in overall talent that you can find a weakness in your opponents strength if you study them enough and put your mind to it. Find their weakness and play to them with your strengths. Finally, work on developing your post play in a two or three man game sense.
Most defenses like to collapse the post area.
Give your team added confidence by working on combination passes between post players and in the post triangle found in most offenses. You don't have to be a great scorer in post to be an effective team in the post. Teach your players to duck to the basket the moment a double team begins down low, or a flare on the perimeter. Teach your second post to duck and replace under as a post player attempts to penetrate the middle of the paint from the post, while your wings dive to the corners. Subtle things like this will also improve your post play efficiency while you work on recruiting and building a more dominant post game.