Student or Learner
‘I’ve handled your work for twenty years and I tell you frankly that I shouldn’t have grown fat on my commission, but I’ve handled it because now and again I like to do what I can for good work. I’ve always believed in you and I’ve hoped that sooner or later we might get the public to swallow you. But if you think you can make your living by writing the sort of stuff you do I’m bound to tell you that you haven’t a chance.’
(W.S. Maugham; The Creative Impulse)
I would surmise that he means "I've handled your work for twenty years, but I haven't grown fat on...".
Is this so? What additional meaning does 'I shouldn't have grown fat on..." convey here?
"I shan't grow fat on my commission." Said when he started 20 years ago, meaning “It’s unlikely that I shall grow fat…”
"I remember thinking that I shouldn't grow fat on my commission."
"I tell you now that I shouldn't have grown fat on my commission" – meaning “It was always unlikely that I would/should have grown fat…”
This is not common at all. You’ll rarely see it these days.
Of course, another possible meaning is, “I shouldn’t have grown fat, [but I did].” But I don’t think there’s evidence for that reading here.