Make it a battle of your against your opponents upper body and you will prevail even against bigger opponents and make it look effortless against or same size opponents. An might be considerably bigger and stronger than you overall, but his arms are not bigger and stronger than your legs. Learn to fight his arms with your legs and you will put yourself on a winning track. You can’t always do it, but when you can it is a truly fine representation of the ideals of Jiu jitsu