When you cast the spell, any creature that is completely inside the cage's area is trapped. Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area.
But 5th-level Wall of Force is vague on this point, saying only:
If the wall cuts through a creature's space when it appears, the creature is pushed to one side of the wall (your choice which side).
But what if you make a sphere or hemisphere, or arrange your 10×10" panels so there's a section where two panels leave a gap only two feet wide? (Assuming a reading which allows the flat panels to be put at angles rather than next to each other.) The wall appears to the left and right of the target, but topologically the creature is facing the same side of the wall (call it the inside) and the caster chooses "inside" to push the creature towards.
Or, as Ben Barden notes, what about a stone wall on one side, and then Wall of Force on the other, pushing into the wall? (This problem would also apply to Forcecage.)
What happens? Is the creature squeezed between the walls? (If so, how squeezed can they be? What if there's only a one inch gap?) Or is that somehow an impossible configuration for the spell? Or is the creature pushed outside the area of the wall entirely (similar to what is described in the higher-level spell)?