65 is probably not too small, unless you are 6' 5" or taller, maybe. Are you sure the ball is completely inflated?
Several websites that sell balls (like www.powersystems.com or www.balldynamics.com , for example) have size charts. You want a ball that will allow your legs to be at about 90-degree angle, with hips at the level of the knees or, preferably a bit higher, when you are seated.
I'm 5'6", and most ball charts recommend a 55 cm. ball for me (some recommend 55 cm for up to 5'5", so I'm kind of borderline, but 55 cm works well for me). If you are 5' 7" or above, 65 cm should be about right.
Thank you soo much, You were right. I didnt have the ball filled up enough. When I first filled it my bottom was below my knees.
I am only 5'4''. So now I wonder if I should go down the smaller size to 55cm. Anyway thanks for getting back to me...