I think it's a combination of things:

1. Type of implants. Rounder ones have fuller breasts.
2. Cost: I've always assumed the tear-drop ones cost more.
3. Surgical quality, which pairs with cost.

My estimation was always that cost was the main component, but there are definitely those whose aesthetics or - new motivation from this thread, desire to flaunt social status - leads to intentionally seeking these types of implants.

The biggest evidence of a botched job is nipple appearance and placement, and scars. Not necessarily the naturalness of the boob job, because some try to inflate their breasts in ways that no surgeon can make it look real.