Photos of Samantha in profile or from the back seem to be rare, but she did always have a nice ass. Just recall her costume from “The Time Machine”. Granted, her backside was not as big as it is in these pictures in her heyday, but she did have a well-sized bubble butt back in the day. Given that she has been out of the spotlight for some time and is now 30, it is not surprising that her ass may be bigger than it was in the past.
Also, it could be the effect of the dress. There are women that have little to no ass in pants that look like they have a bubble butt, if only a small one, in a tight dress because it gives their cheeks some lift. Tight dresses can sometime do for the ass what the right bra does for breasts.