Dunno why it's harder for a woman, in general, to sit a wide draft....but our BO is an avid eventer/dressage competitor.....very slim, limber and agile as she is a competing horsewoman.....and she said riding Epona was painful and threw her center of balance off. She is about 5 ft. 5in.
She said that epona has no withers whatsoever, and that she is so wide that it felt like she was doing a split.
I also DID say my son also complains of thigh and leg pain when riding after a long layoff, so it's not just a woman thing. However, a mans legs are sometimes, oftentimes, longer than a woman's and therefore more able to wrap around a horse with a wide girth. They are also thicker and more muscular, in general.
I suspect alot has to do with muscle conditioning and agility....HOWEVER, the length of the human leg also plays a part. A short person, man or woman, would find it difficult to ride Epona as it would feel like you were trying to balance on a beach ball.....and leg aids would be difficult at best.