Well, the answer is really quite simple. The only difference between a "petite side chair" and a "side chair" (aka regular side chair) is the width. We take this measurement across the front rail of the chair or better defined as the nailhead line.

A petite side chair has a front width of 19.5-inches and a (regular) side chair has a front width of 21.5-inches. That's it, it is really that simple. It does not refer to any other dimension on the chair. The back height is irrelevant, and the depth doesn't change.

So, what's the big deal - for such a tiny difference? You see, the narrower width of the petite allows you to comfortably fit more chairs around a table, while the regular allows your behind to fit more comfortably on the chair. But honestly, they sit about the same. So really, its personal preference.

Petite - 19.5" front width, more chairs fit comfortably around table

Regular - 21.5" front width, more behind fits comfortably in chair