why is there a variation found in the pelvic girdle of male and female where ilium is found to be larger in males and a bit smaller or softer in females?

The Pelvic Girdle, also called the hip girdle, is composed to two coxal (hip) bones. The coxal bones are also called the ossa coxae or innominate bones. During childhood, each coxal bone consists of three separate parts: the ilium (denoted in purple above), the ischium (denoted in red above), and the pubis (denoted in blue above). In an adult, these three bones are firmly fused into a single bone. In the picture above, the coxal bone on the left side has been divided into its component pieces while the right side has been preserved.

In the back, these two bones meet on either side of the sacrum. In the front, they are connected by a muscle called the pubic symphysis (denoted in green above).