In the first place, the only numbers we have are from the Seanchan, and the enemy's count of an opposing army is highly suspect. Historical tendencies aside, throughout the books, mention has been made of the exaggeration of armies and whatnot. Elayne gets an estimation of the true size of the borderland army because she has spies IN their camp. Scouts always exaggerate numbers, through no fault of their own. It is very hard, even for professionals, to estimate the sizes of armies. Finally, we see in the Prologue to tGS that Ituralde is using decoys and civilians in uniform and soldiers in civilian clothes to confuse the Seanchan. Those estimations back in KoD should have been suspect automatically, and we see in tGS that Ituralde's plan has involved deceptions to make the Seanchan move in reaction to forces they believe to be in place, so that they can be attack advantageously by Ituralde's real troops.