Katsu means victory, hence Katsudon is considered a lucky dish.
With regards to Sakamoto, Granny kinda explains it: He's fat now, but still trying to fight like the skinny guy he was in the past. Now she disabled his ability to become skinny, so that he can learn how to fight as a fat guy. I'm guessing the additional weight will give his attacks moer heft, allow him to tank through stuff, that kinda stuff. If they want to go slapstick with it, he could even get some bounciness.
Or maybe when he gets caught by the strings, he'll just suck in all his fat to get loose, then strike back at the opponent. Was kinda expecting him to do this already, would fit the comedy vibes of the series.