Thats the problem, you are taking the "max load" of the Mosquito and comparing it to the "typical" load of the B-17.
B-17F could carry 9600lbs max, but often rolled with 4000-6000 due to range requirements.
Mosquito B.IV carried 4 x 500s as standard load, or 2000lbs on operations, all internal. The 4000 lb load is from a later mark, the B.XVI (Spring 1944) which had an enlarged bay.
The Lancaster could carry 14000lbs max (for standard bombs) but often flew with less than that, again because of range and ordnace requirements of the target.
Besides, the Mossie is in the medium category I think, its kind of apples and oranges.