Otiluke's rugged dial He says:
The sphere does not weigh and is large enough to hold the creature or object inside.
Although not explicit, nothing in this description implies that the size is static and that the sphere will not fit a creature of changing size. Affirm that the effect is large enough to contain the creature. The effect of this spell is continuous. If the creature changes, the size of the sphere must also for this statement to remain true.
Therefore, if the size of the creature changes (as long as it remains Large or smaller), the sphere will adapt to the new size. No harm would be taken.