By mobility, I'm guessing you mean movement speed. (Mobility is a feat that allows certain classes to move around during a battle without drawing attacks of opportunity.)
Positive modifiers to speed include the Monk class, the Run feat, and certain magic items: mostly boots, of course, but also a number of misc. items (like
Onglore's Amulet of War, found on the very first level of the Dwarven Dungeons).
Negative modifiers to speed include encumbrance (which is based on strength) and armor; generally, the better the armor, the bigger the movement penalty.
So I'm guessing your strong fighters are carrying most of the stuff you've found, plus are wearing medium or heavy armor. That combination will slow them down every time.
Also, the better the armor's base AC, the more it penalizes your dexterity; as well as the use of certain skills (like Move Silently or Hide). Like everything else in this fantasy world, it's a tradeoff: tanks are well-protected, but they move like, well... like tanks.
