The sphere is surrounded by or protected by a rubber and sponge-like material in order to keep this sole inhabitant safe in the sphere. The situation that may mostly be is perhaps rolling down a long hill which may be really bumpy and steep.
In these circumstances, the material being used inside the sphere will absorb the damage from the outside and it will cause itself to expand.
The sole inhabitant, not realizing that the sphere it is in is expanding, can enjoy daily life without having to know all the mess and truths in the outside world.