Initially, my main question was just related to the details of how the molecules expand: do they keep the same shape, or do they genuinely just increase as best they can to fill in the area? (I'm assuming the former, just being sure).
If you use the washing up suds analogy, you can see how there's any amount of different shaped bubbles and looking closer you can see that none are a perfect sphere; more like a semi sphere or less than that.
Basically they're in varied shapes on the larger scale by eye. The reality of the smaller scale could be hexagon shaped but we won't go into that.
Anyway looking at the washing up suds, you know that inside each bubble are denser and smaller molecules as well as what is making up the larger bubble being even more molecules.
It's a massive infinite type molecule stack within a stack and so on.
Actually, this is vaguely related to the filling-a-jawkbreaker issue, I think. If the molecules are, say, balls with one hole in, then it would make sense for high pressure to make some get slowly squished inside others, if they could take shapes other than the overall ball, and would similarly make sense for them to get expanded out. Though in turn, this would mean the molecules could expand to non-ball-like shapes, and the vacuum flask issue brought up earlier seems relevant as the few molecules left would expand and with no shape constraint there'd be no reason for the surface area connecting the inside/outside to decrease.
While if the molecules are trapped in a set shape (I'm using balls based on convention and the jawbreaker analogy, something similar would hold for each shape), regardless of size, then the flask issue would make sense, but the way they combine wouldn't so much, as you'd need them to shrink/grow at varying rates, and when one's inside another you'd need the inside one to shrink.
Imagine having a hollow sponge type ball and many other sponge balls similar to it.
You push on ball into the other and then another ball into the other. You keep doing this until you literally cannot push another sponge ball inside.
That's the end of your energy force.
Along comes a weightlifter who manages to push quite a few more sponge balls into that sponge of yours. He has more energy force.
And so on and so on.
Ok, now to keep those sponge balls from expanding, they have to be under immense pressure, which they are in the weightlifters fist.
Now we need more energy to expand the hand that holds the dense sponge balls. If released with lots of energy applied then we have an explosion of sponge balls (molecules) expanding out of the sponge ball but they would be hitting other like sponge balls and appearing to attach to them as half a sphere on half a sphere connected to half a sphere.
Think of a bramble or a raspberry.
Unless they're all different shapes, but that seems less likely. Or they're cups, cup-shapes might work, if they were perfect hemispheres. Or less conventional means of moleules combining...
You can never get a perfect sphere because all molecules/matter are always attached with no free space.
Ok, that's a lot of rambling/speculation. The gist is:
What shape are the molecules, or do they not have any constant, fixed shape? (Size notwithstanding).
No fixed shape because all molecules are moving. They are always agitating due to always pushing against each other in a resistance caused by all of them trying to expand against each other. The denser molecules/matter will expand into less dense molecules and push them up, creating heat.
The faster molecules are pushed up by denser molecules, the more friction is created, which means that you would see super expanded molecules in a friction burn that your eyes see as fire.
How is it pressure would allow them to combine? Is there a special mechanism, or is it just based on one slotting into another?
Let's take a tree trunk near a metal fence (for instance). At first the trunk grows up and out and gets thicker and more dense by the push from the ground into the atmosphere which is resisting that push into it.
The fence is now being pushed away and over time the fence becomes part of the tree. the tree pushes into it. It does this because the fence created a massive resistance to the trees push on it.
Molecules are the same. The tree and the fence are just a mass of more dense molecules than the atmosphere we stand in.
Feel free to keep digging if I'm not being clear enough.