My experience is that smaller heads tend to dent up slightly more easily: they have less surface area in which to absorb the stresses of a stick hit, and will deform permanently with a somewhat less violent hit than a larger head (especially if tweaked up tight). It's just a function of the modulus of elasticity of the Mylar, and the amount of material available to take the abuse.
It's usually a nonissue, though, because I used to beat on my little 6" boo-bams a whole lot less often than I did the 12" or 13" racks, and I certainly didn't whack 'em as hard. But there are indeed a few valid physical reasons that smaller heads might be less long-lived than larger ones, depending on your playing style and the shape of the tips on your sticks (more oval is better then mor round, to spread the impact over a larger area of the head).
They're all consumables. Like sticks, their lives are forfeit as soon as they come in the door. I'm just glad that I'm e-drumming these days: the dual-ply Hart mesh heads I use are the next best thing to immortal- and it's kinda nice to only have to stock 2 sizes...
