Does having a Mob Larger than (5 X level) helps in any way or its just a waste to have larger Mobs in thousands? i.e. for level 100 will there be any benefit of having Mob of 1000 or it will be same if i have mob of 500?
There is no difference.
The ones you can use are the only ones that matter, the rest are just for show.
//Formidable - DeathWish MGS