Fuji, many of us are "big boned" or just not the "standard" adult imagined with most 4 seat designs which, in reality, are 2-3 seaters unless one talks parents with kids.
Realistically speaking, a "true" 4 seater would require a payload of around 400 kgs plus the fuel necessary for the trip. Not many airplanes I know do that. 400 kgs, that is a standard IATA adult plus his bags times 4.
That means, the typical 50 USG aircraft would need to have a total payload (Empty to MTOW) of around 550 kgs.
The "average" family of four, if calculated conservatively, will be less. Again, 2 standard adults with bags (200 kgs) plus 2 kids with bags (around 60 kgs each) will demand a payload of around 320 kgs plus fuel.
As to the protests of some lady passengers or pilot that they do not fit the description of a 100 kg adult, thank you very much, I fully agree, but take into account their baggage and many do. Apart, many think of a GA plane as their own airbound sedan, what fits comes along.