A 200 lbs fat boy will have more buoyancy than a 200 lbs solid body builder. Fat floats, muscle doesn't. How much difference in buoyancy, I don't know.
What makes the biggest difference, based on body composition, is the
volume of the body for a given weight (mass). It's simple Archimedes principle.
A more muscular person, at the same weight, displaces less water than a more fatty person. This is because of the relatively density (volume versus mass) - muscle being more dense than fat.
Also, body density/composition can directly influence the
size of the wetsuit worn, which has a much more direct impact on buoyancy. Obviously, the volume of an individual influences their
surface area. Cover that surface area with highly buoyant neoprene and we can see a definite variation in buoyancy.
Obviously, there are other factors that dictate the size of a wetsuit needed. Height, for example, may dictate a larger wetsuit size, rather than waist or chest measurement.