I believe that the raised health pools in games, like FFXIII and others, is used as a way of controling progression and as a means to separate early game content from late game content. By creating the large health differences, it makes it nigh impossible for low level characters to defeat higher level enemies, effectively forcing the player to first progress through the earlier content.
Meanwhile, after the player has cleared the lower level content, drastically raising health pools (in the course of just a few levels) not only allows the player to contend with the higher level content, but also renders the lower level content far too easy for the player. Thus, players are driven forward from the old, easy content into the new, challenging content.
The end result of this of course is massive health pools by the end of the game.
Meanwhile, after the player has cleared the lower level content, drastically raising health pools (in the course of just a few levels) not only allows the player to contend with the higher level content, but also renders the lower level content far too easy for the player. Thus, players are driven forward from the old, easy content into the new, challenging content.
The end result of this of course is massive health pools by the end of the game.