They get a flat hp bonus depending on their size, like +5, +10 and so on. Don't know the exact numbers on top of my head, but that's the general idea they came up with to increase their hp without breaking the hit dice mechanic.<br/>The difference is tha more hit dice equals more levels - higher saves, attack bonus hp, skill points and feats. Some spells and skills also vary depending on the HD total of the creature they are used on.<br/>A flat hp bonus on the other hand does just that. Keep the guy in combat longer without any other side effects.