why is the size of ova bigger than that of sperms ???

The egg is much bigger because it contains the food store (nutrients) for a possible embryonic development. The sperm is smaller since it only needs to carry the genetic material and the mitochondria for providing energy for its movement. This size difference occurs during gametogenesis (oogenesis and spermatogenesis)

Eggs are larger in size and less in number than sperms. Eggs have larger cytoplasm than sperms. During oogenesis, the primary oocyte grows much larger in size and completes meiosis I(unequal division), forming a large secondary oocyte and a small polar body. Secondary oocyte undergoes meiosis II forming large ovum and a small second polar body.

However during ?spermtogenesis (in males)?, formation of sperms does not lead to formation of any polar body. ??A?l?l? ?t?h?e? ???????????sperms ?f?o?r???m??e?d? ?a?r?e? ?e?q?u?a?l? ?i?n? ?s?i?z?e?.? ???????????????????????????????????????????