There is always a possible problem with any cham. The runt is small, it may stay that way, may grow to normal, may grow but just never be as large as the others. Or is could mean there is something very wrong, and the cham could die. That is something the breeder will not know, they are just being up front and letting you know as much as they know. I like that in a breeder. At least you know what you are buying before the purchase, so you can decide what you want to do.
I'm with Laurie, if the breeder tells you that, that's good.
Sometimes, they just don't develop as large, which makes them smaller. Like Laurie said, it could either be no big deal, where they grow to the same size or they're just smaller but still healthy; or, it's something bad.
i know with dogs and cats, (which i have experience with breeding / birthing / homing / ext) just because they are a runt as a pup, does not mean they will grow up to still be smaller, i have seen many babies who were the smallest grow into one of the biggest... I figure cham's will be different with how they are, but i just wanted to throw that in there, that just because its a runt is not always bad.
But it is a GREAT thing when your breeder tells you something, that most people are not happy to hear!