All spools have the same size outer/inner diameters -- the center hub changes width to accommodate more filament. The center hubs are also interchangeable, and the side panels do not change between sizes. 

Hard anodized aluminum is thicker than standard anodizing. Thicker than 25 microns, hard anodizing is produced using lower temperatures and higher current density to accomplish higher thickness. 

A thicker surface gives these hard anodized spools a more abrasion-resistant surface, keeping your filament pure.