what is the difference between two pyston engine

The one that's got me stumped, is the difference in range of the 747-400 and 747-400F!Both have the same MTOW, same amount of fuel, same type of engines, yet, the 400 has a range of 7260nm and the 400F only 4445nm? What makes freighters that much less efficient?

Whilst what others have said in regard to payload vs range of these beasties is correct, the 744 also has the option of a horizontal stabiliser tank which carries approx 22,000lb of fuel, which the 744F is not offered with. edit - well, the 744 WAS offered with the extra tank when it was in production. I assume the 748i still has it.Cheers, SLuggy