question Tower shield has a -2 due encumbrance would this still apply to a half giant with powerful build? I would think no if he can wield a sword one size large then him the little wight this shield adds should not effect him that much. right?
Yes, it would still apply. Being larger is not beneficial when it comes to Tower Shields, so Powerful Build doesn't help.
so it is more of a bulk thing then weight. still does not stack in my head. (Yes not all the rules make sense I now still I can argue my side.)
the rule says he counts as large for most things even thou he is medium size. as a medium size person he should get the bounce from the size of the shield yet being more muscle than normal he can handle the weight and with training (tower shield proficiency) should be able to hand the bulk of the shield better then most.
Also a large weapon is vary unwieldy the center of balance would be way off yet the half giant can hand that with no problem.
For Half-Giants, the weapon isn't unwieldy though. It's a racial ability they have that, to use your words, allows them to find that 'way off center of balance'. They have no such ability for Tower Shields.