At first I thought max sanity cap was linked too high intelligence. The key characters like Maxwell and Wickerbottom enforced that assumption. Then we have Wormwood that doesn't display any form of great knowledge, and has a very simple language. How come its sanity is so high?
Another weird candidate is Walter. Despite having a title "Fearless", which could contribute to resolve, he is just a little boy and I highly doubt he is a prodigy to be on par with the old lady. Because of his f̶o̶o̶l̶i̶s̶h̶ brave act, he almost got minced by deerclops.
If bravery constitutes to max sanity cap then why Wortox isn't at the top of the list? The imp seems to enjoy the chaos revolving around the constant, and even managed to eat a former soul of it's master (Krampus). You may ask how eating your master is linked to high resolve? Well, the imp faced a dilemna of either accepting to kidnap the little beefalo or go against the master wishes. If it was weak/average, they would never revolt and it's a mark of bravery going against the daily routine.
So, what do you think max sanity cap constitutes to?