Its actually a bit more complex.Flotation takes up a VAST amount of space.
This picture shows what my boat's PO had packed into every nook and cranny in the cabin, filled all the lockers and screwed down their covers. After spending the better part of a day undoing this idiocy, I did some quick research on the buoyancy of styrofoam.
This amount of foam, right here in this photo, would have been just enough to float my KEEL. Roughly 550 pounds, out of a 2600 pound boat. So figure five times this much to keep the vessel afloat.
You don't have to support the weight of the boat with foam, you have to support only the difference between the density of the boats construction and the actual water it displaces. After the keel, the majority of a fiberglass or wood sailboat is very close to neutral density. Boats require surprising little extra buoyancy to keep afloat.... some racy designs like the First 210 and the Pogo 12.50 have positive flotation built in and you would never know it.