My best friend back in high school was/is LDS and he mentioned to me that the polygamy wasn't as common with the rank and file back then, but it got attention due to the high profile types, like Brigham Young.
Now, maybe you'll correct me on this, but I was of the understanding that it was during B. Young's tenure as LDS President that polygamy was given the "go ahead" and not Joseph Smith.
While it is true that the LDS leaders were much more likely to have plural women & girls (wealthier men who could afford them; plus greater pressure on leaders & future leaders to do so by those already engaged in the practice), that amounted to still a high percentage.
B. Carmon Hardy estimates that during its peak in the late 19th century, 20-30% of Mormons were part of polygamous families. (LDS have often intentionally skewed these stats by counting percentages among men...but, if, say, the average polygamous man had 3 wives...you automatically dilute the %).