T--The quick answer is that there is not enough gravity on Earth to hold that much atmosphere down.

J--My understanding is that the planet Venus is about the same size as the planet Earth. Why is one planet of the same size, able to hold down more atmosphere than the other?

T--There are other answers as well. To get any kind of buoyancy effect from air on something that approximates the density of water (living tissue), the air would have to have at least that density, which would require compressing it to over 12,000 psi.

J--Could you traslate that to number of atmospheres?

T--If I didn't make a math mistake, this is approximately the pressure under 27,000 feet of water.

J-- Is it bouyancy or something else that keeps the hovercrafts 'up in the air' so to speak? How high is the pressure under the hovercraft?

T--Under these kinds of pressures, lungs don't work. Gases are toxic.

J--How can life exist in the deepest parts of the oceans?

T--And it wouldn't affect the size of the Earth even if you did it. The pressure required to compress a mass of rock and liquid metal symmetrically more than a few feet is astronomical. And it wouldn't make a lot of difference if you did.

J--Naturally 'strange matter' and black holes are different. :-)
My idea on this accout is that when the Earth was forming, it was composed of elements that were mostly in a gassious state. Perhaps some of these gasses were trapped inside the planet when the surface cooled and solidified. If that was the case, then the solid surface would be experiencing pressures on both the inner and outer sides. If the pressure on the outer surface is reduced then the inner pressure would excert more force on the solid surface, even if this pressure was not enough to increase the size of the planet, it might be enough to cause shifting in the weaker parts of the mantle.

T--Gravity, measured in "Galileos" (Gals) varies from tens to hundreds of milliGals due to the effects of distance from the equator, mountains, oceans, and other natural variations. This is not very much, as 1 standard gravity is about 981,000 milliGals.
J--Interesting

T--No one is doing the numbers in this. At all.
J--Shame

T--This theory is the sort of thing that sounds good if you are about 12 years old, and don't know how things work yet.

J--I do not claim to be a know it all, although I may be older than you.

T--I have this same trouble explaining economics.
J--I'll gladly leave that area to experts like you.

Cheers
Johannes