Because there was a consistent contrast between her physical stature and the fact that she commanded the room. Not to mention the fact that WOT has a wide range of physical descriptions of people and their native land. All of the Cairhein people were short of stature. It is their defining trait. It makes one wonder if the Aiel will all of a sudden be dusky skinned with dark hair?

For Moiraine, her petite stature even supports a distinct plot point in the death of Lanfear. Moiraine was able to climb up onto a wagon without jostling it at all, and tackle Lanfear through the doorway. She couldn't have done this if she was not petite.

However with all that being said, I think that as an actress, she has the ability to play the part. They could always use forced perspective to make her seem petite (see the first Lord of the Rings). Darken her hair a bit....you're good to go.

~Jeordam