Well WinterKirby, I know HP demonstrates the amount of damage and injury a Pokemon can endure. Not how many hits, because defense is important as well. Blissey has low defense but huge HP so she will only endure 2 hits but practically she will endure a ton of injuries and damage. Shuckle, having huge defenses and low HP, will endure 10 hits but he will be barely scratched. So, HP is the toughness of the Pokemon. Blissey may not be able to last as long against physical attacks due to low defense but she can survive horrific injuries.
And HP is based on body mass in some cases. Different Pokemon get their HP from different factors. Like, Wailord has gigantic HP because he is very large. Wobbuffet and Blissey may get their tremendous HP because of some sort of physiology which enables their body to cope with large amounts of damage. Wigglytuff may just be very tough so it can endure injuries thus the high HP.
Body mass is a factor. It's just not the only one. It's the most logical one though to be honest. A bigger force is required to bring down a large animal than a smaller one. A lion and a cat for example. Or a horse and a pony. The bigger you are, the harder to get knocked out because you have more surface on your body to spread injury on, so you can take more injuries.
Of course as I said, it's not the only one.
I would like to see your point of view. I mean you said HP is the amount of damage a Pokemon can take. Where exactly does this ability depend on in your opinion?