A size 8 in the Sears catalog or website would be a size 8 in the Sears store, it doesn't translate to a size 6 in their store. A size 8 in their catalog will fit the same as a size 8 in their store. Same would be true for JCPenneys, Eddie Bauer, Macy's, Express, Victoria's Secret, L.L. Bean, Gap, Land's End, et.al--the size you buy in their catalogs/online is the size you'd buy from their stores, they are not making a special batch of "downsized" clothes for their stores. I'm a Missy size 8, the clothes I buy from the catalog/online fit the same as the clothes I buy in the store (and I'm a technical designer, so I'm well aware of fit). I've worked for the mass market brands, the clothes that are sold in catalogs/online are the same as the clothes that are sold in stores.
A petite model is a size 8, because that's the medium size. A petite model is smaller, in height, and that's what's accounted for in the fit.