My expertise in history lies primarily in the history of costume and dress, so I thought maybe I'd give this a go. I'm new here, so be gentle. From a fashion standpoint, butts have gone in and out of style just like anything else. (I feel a little silly discussing this, but nevermind.) (Also I'm assuming here that you're referring primarily to men in the United States, of course fashion in other countries would be different.)

In the late 1920s, a tube-like, curve-free silhouette was in style, and women wore undergarments to flatten out their entire torso. This is partially in resistance to their mothers and grandmothers, who only a few decades earlier in the 1880s added bustles and ample amounts of ruffles, pleats, and bows to their backsides to create a look that definitely emphasized their rear-ends. Their undergarments also helped to pinch their waist in to make their hips and backside look bigger. Also, if you look at nudes and erotica from that era, you can see that curvy women, to the point of being what some would call overweight by today's beauty standards, were considered sexy. Nudes from the 1880s-1890s are generally very curvy type ladies. Curves were undoubtedly considered feminine and attractive by both men and women. Slightly later, in the first decade of the 1900s, a corset that forced the figure into an S-shape was popular (The Gibson Girl look) that really pushed the wearer's butt out in the back in a very uncomfortable position, but was wildly popular.

Again in the 1950s, the popular silhouette emphasized the hips, both in full skirts with a pinched waist and slim ones which fit close over the body. A curvier girl was luckier in this era, if she wanted to fit into the popular look of the day.

If you want to go back even further, look at what ladies were wearing around the 1770s and see how it emphasized their hips and bums. Skirts with "saddle bags" underneath made a comical depiction of the female figure, sometimes with hips being twice or three times the width of the woman inside the garment.

This is all based on fashion of course, and how men actually personally felt about butts during each time period would I guess be up for debate. Maybe someone knows of some poetry or literature that praises the posterior, so to speak. But going just from the standpoint of popular fashion you can see that there are definite time periods where being well endowed in the bum department was desirable, and other times when it was not.