A big body is a liability for climbers. Indurain was a big guy and wasn't a natural climber but he was so damn strong that he could power his way up the big climbs.
In general bigger guys have more muscle mass which is why they time trial so well. Natural climbers tend to be smaller guys but it's rare to find a climber who can time trial with the Indurains and Ulrichs of the world.
When you look at how strong Indurain was, frontal resistance really isn't an issue.
To win the TDF you HAVE to be able to time trial well and you have to have enough brute strength to make it over the mountains with the natural climbers.