You're not going to believe this, but a half-ogre might work, depending on how you interpret Giant Blood.
There's a Druid 7 spell in Eberron Campaign Setting, Return To Nature. It has various effects depending on creature type. Giants who fail their Fortitude saves become smaller, as though affected by Reduce Person (Str -2, Dex +2, instead of the usual size change effects). It's instantaneous duration, which means it never wears off, can't be dispelled or turned off by an antimagic field, and you can cast it repeatedly with accumulating effects on size (but not on ability scores, as a size bonus or penalty won't stack with itself).