In my games, Constitution usually gets the second highest score, except for Clerics, Monks, and Paladins, where it's usually the third highest.
Also, my players invest in items that enhance Constitution as early as possible to give the most hit points.
Especially for Wizards, this is QUITE in-character, considering how smart they're supposed to be.
A common Level 20 Wizard could get about 3 hit points per die, have a 14 Constitution, +6 enhancement, and also get a Tome of Bodily Health.
This is 150 MINIMUM for most people who roll better than crap. There are usually more items involved, such as a Vest of False Life, and Toughness. (I have House Ruled Toughness a bit, but not enough to really add too many hit points.)
Anyway, Fighters usually have over 300 at this point.