Cortez
Tough but Fair
If the bit is fat at the edges, it could be stretching the corners of the lips not actually pinching them, I wonder if this is the case as you say he is a cob and cobs tend to have quite fleshy lips/mouths. Ive recently come to the realisation that with my boy, I can use any sort of fat bit in his mouth. I was trialing him in a continental gag for jumping, i hacked him on for a while to get him used to the bit and noticed the the corners of his mouth got split, they did not heal, vasaline didnt help, I let it go for a few days to see if it would settle and then one day i tacked him up, went to get my hat, came to get him out of his stable and his mouth was already splitting and weeping, it was very clear that the bit was stretching his mouth at the corners. Much as i get the fact that fatter bits are supposed to be kinder, for my boy they certainly are not, its another bit to add to my list of redundants!
Actually I see that latest research has concluded that fat bits are NOT kinder and the narrower, thinner ones are less uncomfortable. I think it depends on how much room there is in each particular horse's mouth. Just a thought: does the OP's cob have pink lips by any chance?