The Fireball Inflicts Normal Damage

Although its burst is constrained by forcecage, the fireball inflicts normal damage. The enclosed space doesn't matter. The fireball's burst has merely encountered an obstacle.

It Wasn't Always This Way...

In the Player's Handbook (1978) for Advanced Dungeons and Dragons the spell fireball has a burst that

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 covered by the fireball is a total volume of roughly 33,000 cubic feet (or yards)]. (73)

Many DMs interpreted this to mean the area of a fireball could fold in upon itself in a small enough area, dealing damage multiple times to foes in tiny rooms, making fireball a much deadlier spell both for monsters and careless magic-users. Similar language exists in the Player's Handbook (1989) for Advanced Dungeons and Dragons, 2nd Edition. This language is absent in Dungeons and Dragons, 3rd Edition (2000) and many of its iterations, including Pathfinder.

...But It Can Be That Way Again

If homebrew is acceptable in the campaign--and nostalgia permeates the gaming group--there's Frank and K's prestige class seeker of the lost wizard traditions (available here and here and here). The class's 1st-level class feature spell reflection makes a D&D 3.X character's lightning bolt spells behave like 1E and 2E lightning bolt spells used to (i.e. bouncing off surfaces), yet it's not until the 3rd-level class feature burst conservancy (acquired, at the earliest, at character level 9) that a character's fireball et al. spells start folding back on themselves. Although sometimes considered a joke by critics, the class is playable and entertaining. Old-school gamers may enjoy the opportunity to play 3.X (and, by extension, Pathfinder) casters with 1E and 2E flourishes.