Deadeye,
I have to disagree. If I took you out into space, and threw you out of the space ship without a suit, you would probably explode, or at least balloon up quite a bit.
Why? Your internal pressures would be much larger than the external pressures, that is…space is mostly a vacuum, therefore very little external forces to counter balance your internal pressures.
Conversely, if I took you and shoved you out of a deep sea sub, at about 1 mile down, you would be crushed. Your internal pressures are far less than required to balance the external pressure.
The forces generated by pumping are internal pressures minus external pressures, produced the the vacuum pump, equalling a net positive outward force.
Blood trapped in the penis during a normal erection ideally produces a hard erection, but not enough to cause expansion beyond its normal limits.
However, if you reduce the external pressures that act against the erection, you get a relative increase of internal pressure. This increased net pressure can be enough to cause expansion.
There are more than a few pumpers who have achieved excellent growth with pumping alone.
Sparkyx