When editing a mob's health in an entity tag, such as in the following command:

/summon Skeleton ~ ~ ~

{Health:30}

The mob's health will not be increased beyond its normal 20. The mob's health can be decreased, but as soon as its health is set to a number greater than its normal maximum health, the new health will not be set.

Even when a mob is given a health boost potion effect, its health will not increase beyond its normal health.

The maxHealth attribute will not increase the mob's health either -

/summon Skeleton ~ ~ ~ {Attributes:[

{Name:generic.maxHealth, Base:25.0}

]}

The Skeleton's health will not be increased.