Because they're smaller, so they'll be harder to hold onto, and therefore they're making the not necessarily correct assumption that one's kegels aren't as strong as a younger woman's. If you have something larger and it's too large, by all means try the smaller size.

I believe they are saying that women who have given birth may have more 'stretch' to their vaginas due to less muscle tone. If that is the case, someone would do well to buy the larger beads, work their muscles, THEN go down to the smaller beads.