What is the reason for the difference in grave depth between males and females?

It is desirable to deepen, widen, and improve the grave, whether for a deceased male or female, old or young. Most scholars have deemed it desirable, while some have considered it obligatory. The intent of "obligatory" here refers to what ensures proper burial and protects the deceased from predators, and prevents the spread of odor. This is determined by need and not by a specific limit in Islamic law. Anything beyond that is desirable. There is no distinction in this regard between a man and a woman, or a child and an adult.