The problem is that you have no context or perspective on the model you've seen a static image of.
1) Standing next to the full-sized, animated model will give you a very different perspective when you're looking up from his toenail
2) Knowing that he will only come out of his lava pool to reveal his full figure in a Heroic-only phase of the fight will give you a bit more context
He's not just going to be running around the moment every raid reaches him. :)
All that stuff about perspective is well and good, but it has absolutely nothing to do with the CONCEPT of Ragnaros, an elemental being, being fashioned with / creating legs for himself.
He's an incarnation of fire. Legs, especially those designed in the bipedal fashion, are extremely awkward for anything with an alternative.
Why do elemental lords have to look more and more humanoid? First there's a being of wind wearing turbans and armor that looks like it's made of cloth, and now Rag is using legs to travel around and risk tripping opposed to, I don't know, RIDING A WAVE OF MAGMA?
But then, we're talking about the same art team that supposedly thinks Moonkin are a "cool" looking form for every Balance druid to be perpetually combat-stuck in, so...