To borrow from a comment I made a while ago:
r/smalldickproblems is depression fuel; guys lamenting about having tiny penises and saying that they will never be able to pleasure a woman. Those from averagedickproblems and bigdickproblems will often go into this sub and tell them that it isn't so bad and that they shouldn't focus so much on their penis size; very insulting.
r/averagedickproblems is full of guys with regular dicks who are either pornsick or feel ashamed for not being above average. They lament about never being able to "fill" a woman up in the same way that a larger penis does, and that they'll always be subpar. Those from r/bigdickproblems often brigade this sub and try to say that the average guys have it better than them as they don't face any difficulties in the bedroom; this comes off as bragging.
r/bigdickproblems is full of braggers who complain about visible dick prints through their trousers, of being too big for blowjobs and having difficulty putting their dicks inside a woman. This sub has a heavy female presence, of course, as they are obsessed with the men on this sub. Bunch of unashamed braggarts, essentially.