The burst of the fireball does not expend a considerable amount of pressure, and the burst will generally conform to the shape of the area in which it occurs, thus covering an area equal to its normal spherical volume. [The area which is covered by the fireball is a total volume of roughly 33,000 cubic feet (or yards).]
Gary Gygax's crystal clear prose aside, what this means in practice is that a fireball is normally a sphere of radius 20 feet (in a dungeon; for some reason, above ground that changes to a 20 yard radius... ah, those first edition rules...)**. But if you don't have enough space to fit in a sphere that big, the fireball spreads out further until it fills the same volume as a 20 foot radius sphere. Which means that if you're in a standard 10 foot wide and high dungeon corridor (as they all seemed to be for some unfathomable reason*), the fireball will spread along the corridor for a total of 330 feet!
Yeah, sadistic Dungeon Masters used to enjoy using that fact to screw up plans of the adventuring party.
Another consequence used by even more sadistic Dungeon Masters is that if the fireball went off in a confined space where it couldn't expand to its regular size, the compression would increase the damage by a factor equal to the 20 foot radius volume divided by the actual available volume...***
If you knew this and ever either perpetrated it on players or fell victim to it as a player, consider yourself a true Dungeons & Dragons nerd.
* Okay, it's not unfathomable. Dungeon corridors are that size so gelatinous cubes can fit perfectly in them.
Notes added later:
** Boy oh boy, is this bit confusing. In Advanced Dungeons & Dragons (first edition), there was a rule that the ranges of spells were listed in "inches", and meant to be interpreted as scale inches (i.e. inches on a map that you used to lay out combats). Now, underground, a scale inch was equal to 10 feet, but above ground a scale inch was equal to 10 yards, or three times as big. With me so far?
The area of effect of spells was also listed in "inches". However, in this case, the scale to use was always one inch representing 10 feet, no matter where you were. So the range of a spell would triple above ground compared to in a dungeon, whereas the area of effect would not. Still with me?
However, the actual text for the fireball spell that I quoted above is directly from page 73 of the Player's Handbook and, as you can see, clearly states that the area of effect of the fireball is expressed in "cubic feet (or yards)". This is an outright rules error in the Player's Handbook, and led to my own confusion when writing the original annotation for this strip.
*** No, this fireball compression leading to damage multiplication was not an official rule. But a lot of Dungeon Masters used it as a house rule. It was even recommended in Dragon Magazine once as a way to catch players unaware...