Aside from the obvious issues of mass-shifting, the problem with calculating Fort Max's size based on the Spike headmaster is that it doesn't account for the size of his towers from his "city mode"...
But that's okay... city modes always suck when it comes to keeping any sense of proper scale.
Actually I kind of like the idea that Max would only be around that size... I remember him best from Marvel and IDW comics, so I never really needed to see him as Metroplex Part II.
Also, just for the sake of trivia, according to Devastator's original G1 bio, he's only 60 feet tall. So by your calculations, Fort Max would still be a bit taller than Devastator, which is respectable.
zmog