I think the size of ball you use depends on your height. Maybe the ball you have is too small for your height. Just a thought.
That's what I'm thinking I need as well. The 65 doesn't give me the height I need to do the ball work in XTrain.All the charts say that I should have a 65" ball(I'm 5'8"), but I feel much better on a 75" ball so that's what I use.