Lol, If you've ever looked at Magus, he's pretty beefy, like Crono nad Ayla Oo.
Anyway, survival of the fittest isn't just about power, but also intellegence.
And surely, you can't say Lavos is not fit when he has the power of the whole planet, and a bunch of kids finished him off, whom one was deleted and one was abducted and killed.
In proper perspective, Lavos is the one who should have won in the end if we had it all accounted in "survival of the fittest". The game makers wanted Crono and co to win, so they did. End of story.