Ronald, as Vaughn said, the pressure differential is zero. If, instead, you were talking about a situation where the foam sheet were a diaphragm with a pinhole and if there were some leakage through the bowl then, yes, there would be a difference in pressure. Also, I think that you are confusing pressure and force. For instance, a 1" diameter vacuum chuck and a 6" diameter vacuum chuck would have the same pressure differential between the inside and outside of the part of the bowl in contact with the chuck. However, the force is considerably different. Suppose that the vacuum that you have applied is 10 PSI differential (vacuum is typically stated in terms of inches of Mercury -- in-Hg, but never mind that minor detail -- 10 PSI differential is approximately 20.3 in-Hg) below ambient (ambient is the current atmospheric pressure in your shop). For a chuck with a 1" interior diameter, the force would be about 8 pounds force. For the 6" interior diameter chuck, the force would be a bit more than 28 pounds force. Just for grins, suppose you had an 8 inch diameter chuck and applied the same amount of vacuum, the force would then be just over 50 pounds force. Bigger is not always better!