Droomph, I got to say your opinion on HP is so good! 'How ready they are to fight'. This is so logical! It kinda links to willpower also except than mass alone. If you shoot a Jigglypuff and a Wailord once at the same area Wailord, being bigger, won't bother as much thus it will be able to keep fighting. Jigglypuff on the other hand will be quite crippled. Chansey has so much HP because of her size but also because of her willpower to keep fighting. It's tenacious of life. I think HP is the package that consists of mass, willpower and kinda pain tolerance since if you are a weakling and can't take pain then you won't be willing to fight after a broken toe :D
Now it makes sense. I love you!
EDIT : I came up with another idea and I want to share it.
Def/ Sp.Def is how much protection your vital organs have and how much damage your body can keep away from them, while HP is how much direct damage your body and vital organs can tolerate.
Def is not about protecting the body but the organs. Most Pokemon have high Def because they have a sturdy body that protects what's in the inside. Besides, it's the vital organs that have to be damaged in order to kill someone. I could shoot you all day long on the legs and arms and as long as I could stop the bleeding you would never die.
HP is the ability of the Pokemon to survive and tolerate vital organ damage. This depends also on mass and size because the larger you are the bigger your organs are thus the more damage they can take. HP is about how robust your body and organs are.
Chansey and Blissey may not be huge but their vital organs are large and very robust thus they can tolerate insane damage. They have low Def because nothing protects their organs except from their soft, plushy body.
For example, Miltank has dense muscles, thick skin and fat and heavy bones. This is why she has a great Def. Because her organs are well protected by her sturdy body. A punch won't do much against her because all of the shock will be absorbed by the muscles and her organs won't be damaged. Blissey doesn't have protection for her organs so a punch would directly damage her vital organs because nothing protects them or absorbs the shock. But, due to Blissey's robust organs she can tolerate it and keep on fighting.
If Miltank and Blissey suffered exactly the same damage to their organs then Blissey would keep kicking while Miltank would faint. That's because Blissey has higher HP and can tolerate more injuries.
E.g.
Two guys have equally weak abs thus they have the same low def. You punch the first one and passes out after 1. The second one tolerates much more and faints after 10. The second clearly has more HP. The punches hit directly the organs because no muscles are there to absorb the shock. So, the second guy can tolerate more damage to his body and organs because his organs are more robust.
Something like that. Agree?