I think the giant's Fort save should be +15. And they get low-light vision and darkvision, FWIW.
As for them being close in power. . . not that close, from where I'm standing! If they were both greatsword wielders, for example (a pretty typical Fighter build), then while the human could have say, Greater Weapon Specialisation, and so - without taking magical/unusual items into account - might be doing 2d6+7 damage, with whatever Power Attack might provide as well, the giant might be doing 3d6+15, with the same PA potential of course. Without PA, that's an average of 14 vs. an average of 25.5, per hit. Then there's reach. . .
Comparing all those extras, including fairly crazy stats and insane armour, to 4 feats, 12 skill points, and access to higher level Fighter feats. . . it doesn't really seem like a competition, even.
Giving the giant gear as if they were a few levels lower is an interesting idea. . . but I'm still not sure that would balance things out for the ppor old human here.
Or am I missing something, regarding all these numbers and stuff?
I'm with you on this. The giant in the example has a lot more bonuses to stats, natural armor, reach, etc. that make it way more powerful than the human in the example. What my group does is simply this: if a player wants to be a monster we look at its LA (minotaur for +2, for example). If the party's average level would allow for a character with a LA of +2 (meaning the party is level 3 or higher) then the player makes a minotaur character who has 2 levels less in a heroic class. He/she doesn't get any of the racial HD, feats, skill points, and such but does get the special abilities, stat bonuses, skill bonuses, etc. I've found that this works out pretty well and would allow for people to play things like Trolls at 6th level instead of 11th, or Stone Giants at level 5 instead of level 16.