Other than form factor, is there any practical difference in sound or performance between a mini/single-coil-sized humbucker and a regular sized humbucker?
In short, yes! There are tonal differences between the two. Minis will have more mids and highs than the regular humbucker. This is due to two reasons. The first is the width of the pickup. The small width means that it will pick up a shorter length of the string, which translates to shorter frequency wavelengths (higher frequencies). The other factor is the core of the pickup. The iron core is smaller in the minis and this results in a little less output and bass. There are many variations on the regular humbucker and the mini humbucker. This article points out the differences between some of them. http://www.premierguitar.com/Magazine/Issue/2009/May/Humbuckers_and_Mini_Humbuckers.aspx