I understand that a western style of blade is meant to fix the point and chop behind it while a santoku is intended to slice with.  I also find the width somewhat useful for carrying cropped ingredients around. But what is the main purpose of the blade vastness? Why not skinny(following image but curved)?  
The wide and long blades of western and santoku can BOTH be used for both slicing and chopping. The main difference between the two of them is the tip. Slim/skinny blades enable them to slide through the foodstuff with reduced drag and greater mobility for turning within a foodstuff (eg. breaking down a carcass, or filleting a fish). On the other hand, wide blades provide greater stability and balance required for safe fast and precise processing of foods using chopping and slicing techniques. Both of these can be evidenced by trying to fillet a fish or breaking down a cow, and by chopping food. If one tries to fillet a fish with a chef knife then they should notice considerably more drag preventing fast and efficient processing, conversely if one tries to quickly slice a large food item with a filleting knife then they will find the food catching as the knife is repeatedly raised in quick succession with each slice, and lack of consistency with the precision of slice thickness. 