Simple - it's how we evolved. Men by and large are physically larger and women smaller and both genders find this sexually appealing. Exceptions to this are rare. It's little to do with "double standards of gender" and more to do with what they know their playerbase tends to find appealing for each of the two genders.
That said, I view the extent of the dimorphism with Au Ra to be a mistake and I would not want to see it repeated with Viera. They can just do what they ought to have done with Au Ra females - allow for a bigger height slider that lets the men go both below and above the female height, problem solved.
Also, as was pointed out earlier in this thread, female rabbits are only "larger" to accommodate sexual reproduction, i.e. possess more body fat, but are not longer, per se, and it's also not uniform across all leporine species. However, Viera are not rabbits (or hares, for that matter) and just loosely rabbit inspired, so it's of little relevance how they differ in size as animals.
