[erroneousfunk]

Known genes associated with hip bone size in women (but not men) are found on chromosome 2 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2522269/). But the expression of those genes is likely controlled by estrogen (http://www.sciencemag.org/news/2016/04/woman-s-pelvis-narrow...)

Individuals who have androgen insensitivity syndrome (XY chromosomes, but that don't respond to testosterone and are, for most intents and purposes, sterile females) will have large hips/pelvic bones.

So, yes, hip size takes a few modified base pairs (although probably all within one gene, or pretty close together on the same chromosome), but evolution only needs to "decide" (heavy emphasis on the quotes there) that this physiological change is hormonally triggered. Many skeletal changes are hormonally triggered, so this may have even been the easiest/most likely path.