Osa-Stronk!

It's true in a way. There is a mental limit on strength that's there to keep people from fully using their muscles, because you can do serious damage to yourself with your full power. With training or will power you can extend that limit a bit... with adrenaline and complete disregard for what the lift will do to your body, people have been known to lift cars (this is known as hysterical strength... note that adrenaline doesn't actually add strength, that's already there in the muscles, it just unlocks the ability to ignore limits and use it).

I think it's much less complicated than that. Maybe. The combined weight of the tub and Wakasagi was too much for Osa-Kagerou to push along the ground and that's what Osa-Kagerou thought too; but, when Wakasagi claimed to be as light as a feather, Osa-Kagerou could respond by picking up both her and it effortlessly.

Osa-Magic!

Nope, that's exactly the same. Once belief that it could be done was established, the impossible became easily possible. Much like how the four minute mile was considered impossible, but once one person did it, many people suddenly found they could also do it. All that was needed was to overcome a psychological barrier that was limiting performance.

Itt: attempting to quantify Osa-yukkuri !logic.