I don't think freighters particularly do gross out more than pax aircraft. Yes they may well do so in one direction, but because freight is by definition a one-way load rather than the two-way flow you get (eventually) with pax, they are left finding backloads which often come in well under gross.
Then you get lightweight loads that cube out (once read of about 15 tons of shaped polystyrene foam that filled a DC-10), and freighters also do a lot more completely empty positioning to get to where the load is coming from. And their utilisation in hours per year is invariably way below their passenger cousins.
Meanwhile look at all the pax 747s operating through Heathrow, especially to/from Asia, they often seem to be up to gross in both directions.