Re: Who would you rate higher : Tim Witherspoon or Jim Jeffries ?
Than whom? Every good fighter was small compared to Jeffries and so would have been later champions such as Dempsey, Tunney, Schmeling, Charles, Marciano, Patterson, etc.
Being big is not something to criticize in heavyweight boxing. It is just a fact of life. Jeff was probably the biggest top heavyweight prior to Foreman (or perhaps Ali).
Peter Jackson was actually bigger than the champions I listed. Corbett trained his weight down for long fights, but he was taller and rangier than the men I listed above.