While an older source, the Diablowiki site has some good references from development plans for health orbs.
The intent was to have a variety of different strength orbs in game, ranging from 15% to a rumored 100% (the following was on the official Diablo 3 website before they took down the game guide for revamping):
More powerful health globes, capable of completely restoring your health even at death's door, might exist somewhere in the world of Sanctuary. Of course, that's probably wishful thinking.
During a demonstration of the still-in-development PvP Arena system during Blizzcon 2010, the health globes that spawned provided a 40% heal.
In addition to the variable strength of the orbs, there are item modifiers that provide a direct increase of the amount healed. The amount provided varies widely, and scales with item level.
Some examples I have seen are a level 10 rare item that provided +40 life from health globes, a level 8 rare item that provided +28 life from health globes, and a normal level 10 magic item that provided +17 health from health globes.