Way an egg is so much larger then a sperm cell, need .

The egg contains both food and mitochondria in addition to its vital DNA, thats why it is much larger. The sperm cell has to be much smaller than the egg cell, because it is the one that swims. If a sperm cell were as big as an egg, it would not be able to move very fast nor very far.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.