When perk farming, you are looking for get as much perks in as little time as possible. 20 perks in one sortie taking 60 minutes would be less efficient than making the same in two sorties taking a total of 40 minutes only.
So a bombers rank as a perk farmer is depending on bombload, OBJ (which determines the perks per destroyed object) and the ability to apply that bombload (getting to targets and most preferably getting home as well for the 25% perk bonus).
The He 111 has about he same bomb load as the B-26, but only a very slightly higher OBJ - 23 vs 20. So both bombers can get about the same number of perks for bombing, but the B-26 is much faster and better armed, it has a much better chance to survive the whole bombing run.
A G4M has only about half the bomb load, but almost twice the OBJ, so it has to bomb fewer targets than the He 111 for the same perk gain. On top of that, the G4M can quickly dart to altitude, it can rocket to 20k, drop on one or two front bases and be home while the He 111 is still struggling to get any altitude.
Just for the record, the highest perk gain I ever got for a bombing run was 110 perks in the Ki-67. Of course at that time the perk bonus modifier was quite high, you can't get that under normal circumstances.
