Deadlifting helps developing a strong back. As heavy a weight as you can move for five reps. Squats, too. A stronger body is going to work better on any bike and it doesn't really take much to make a difference.

The other thing is your position on the bike but that's kind of hard to really judge; if you just pose to take a picture it probably won't be quite the same as when you're actually pressing on the pedals with some force. Videoing yourself on a trainer is perhaps the best way to see what's going on.