Even if you wanted to argue that, muscle strength is different than using Ki strength to increase your strength. Goku likely maxed out his muscle strength, which is why he was forced to master Super Saiyan and increase his amount of transformations. Whereas Gohan, became extremely weak and rusty (muscles included), but he did no Ki training during his Z-Sword training. He may be able to lift more than Goku by sheer brute strength, but when it comes to how powerful they would be, Gohan would come up on the short end of the stick.
Q. What's the trick to winning battles [in the DB world]?
A. Toriyama says it all comes down to 'ki', the size of it and the ability to control it. He says it includes several forms: genki, yuuki & shouki (energy, bravery & true character). You can only train your muscles to a certain point, it's 'ki' that carries you beyond that and what helped Goku come closer to being the strongest warrior in the universe.
