I got my hound dog's weight up to 36 lbs using several effects, including well-fed from the moveable feast (also a pith helmet, wearing a feast, sympathy, empathy, leash). Mafia correctly displayed its weight as 36 lbs, but it only counted +4% combat rate from the hound (checked with "modtrace combat"). Adding on another buff (pool weight) caused mafia to show it giving +5% combat. Also, "modtrace weight" doesn't show the feast buff, which is probably related.