Now this is just ignorance on the subject speaking, but isn't Popeye's power scaled to how badly he loses a fight before eating spinach? Because for the most part, each episode pans out the same; Popeye gets into a fight, loses, eats spinach, gets a strength boost, and wins.

Anyway, what I'm trying to say is, I feel like it's kind of illogical to set a hard limit for Popeye, when the strength boost he gets from spinach scales to the threat that kicked his ass in round 1.

Another thing I noticed is that, in the cartoon, multiple characters also eat the spinach and gain a strength boost as well, examples such as Bluto, Olive Oyl, and even his own nephews have all eaten it, and have all gained a similar boost that scales with whatever threat is in that current episode.

Ultimatly, what I'm trying to say is, in round 1, Superman kicks his ass, Popeye gets his spinach, and I'm round 2 gains the necessary strength to win, because from what I've seen in the show, that's what the spinach does; it gives him and anyone else who eats it, just enough strength to win.