You have a bolt with some size head on the outside, and a space underneath it which you're going to make more and more like a perfect vacuum. This means you're reducing the amount of air in the space.
If you remove no air, there's a bunch of air molecules whizzing around bashing your bolt head from underneath, and from the outside.
If you take half the air out, there are less air molecules bashing your bolt from underneath, but the same as before from the outside. It is harder to pull the bolt out than before as this 'outside' air keeps bumping into your bolt head, keeping it pushed in like before, but you have less help from inside.
If you remove all the air from the space underneath the bolt, now it's harder again as you have the same outside air bashing the bolt, and no help from underneath.
If you make the space underneath bigger and have no air in it at all, you still have no help from air bashing into the underneath of your bolt. Whatever shape or size of hole you make, it makes no difference - no air means nothing bashing into the underside. Meanwhile, outside the same 'outside' air is bashing into the bolt head. So it stays pushed in to the same degree (i.e. air pressure times how big the bolt head is), with nothing helping you pull the bolt out from underneath.