Oh good, an exam!
Here are the facts: fast light LB's SB-2 and Pe-2 have 6 x 100 kg bombs, whereas the IL-4 and DB-3B have 10 x 100 kg. TB-3 have 250 kg. F/B's have 2 x 100 kg and are probably irrelevant unless you're farming up XP/morale for running missions on them. A few rare ones have 500 kg bombs, and all have some sorts of machine guns.
The knowledge that's been touted around is that heavy bombs with larger blast radius are better against dense terrain or forted units, whereas light bombs have lower radius, but you're hitting more units. This suggests that you should use IL-4/DB-3Bs against open targets. In practice, these level bombers regardless of miles flown tend to fall off in terms of bombing kills around 3-5 sorties in.
I have suspected that this was an issue with supply, which in 12.xx is drawn from the closest base and then, supposedly, from other depots on a per-need basis. It's also possible in this black box that it's better to spread out your 14 regiments over 14 (or 7) airbases, and they will draw supply separately and without overdrawing from one source. This is conjecture, but it is the only other factor that you could be alluding to in the past few messages (ruling out low fatigue, miles flown, enemy interception and escort, sortie range, AA pathing, morale/experience, guards status, rail need).
Edit: also empirically, the heavies do much better against airbases. Could be due to less AA turn-arounds due to their higher endurance.
I hope this is enough for at least a B+.
< Message edited by eskuche -- 6/22/2020 10:40:09 PM >