Actually being huge can be a problem to climb mountains, because huge are usually heavy and mountains aren't that stable.
Opera makes a really good point here. There's a very good reason why large animals display no mountaineering ambitions here on earth. This reason can be neatly summarized as SPLAT! Mammoth trails go around mountains or through valleys. Also, huge units should not be able to be loaded onto boats.
Mountains in Civ4 are supposed to symbolize terrain too hazardous to traverse. Large mountain chains like the the Alps or the Andes would be best symbolized by mountains mixed with hills. Preferably mountaineering would only be given to a select few units to symbolize either super expert knowledge (like say dwarven recon units) or supernatural ability (like say incorporeal undead). A "mountain trail" improvement might give a solution everyone is happy with, in that you can make mountains traversable by investing resources for this end. Heck, dwarves might even get a unique improvement called "tunnel" which is a road only they can use. Could even go under shallow sea tiles to connect islands. Obviously a late game tech if possible at all.
What makes strategy games interesting is when they force you to make hard choices, but not so hard that it's simply an exercise in frustration. Rife is getting there step by step.
Thanks for sharing this module Milaga.