Pride is -1 stomach capacity, gluttony is +2 stomach capacity. This means with neither skill, you have 15 fullness available. With just Pride you get 14 fullness available, with just Gluttony you get 17 fullness available and with both you get 16 fullness available.
At this point, if you have both skills, and eat 12 fullness in mafia's interface and another 4 fullness in the relay browser, mafia remains at 12/15 fullness instead of updating to 16/16. It remains that if you try to eat a 1 fullness food, mafia will update to 15/15 fullness.