Noroma Mahou to Yobareta Mahoutsukai wa Juuryoku Mahou de Musou suru - Vol. 2 Ch. 10

[cryum]

weight as we feel it comes from the pull effect of gravity accelerating us towards the ground.
we get lighter or heavier depending on how strong we get pulled and how much mass there is to pull.
so by my understanding his magic so far is only amplifying or negating the gravitational pull, not the mass.
i am quite sure of that because he can float in place when lightened and doesn't naturally float upwards when weightless.
i have read another manga that had weight manipulation as the MCs magic and have seen some videos detailing such effects, so i am familiar with the differences of weight and gravity effects.

but i am willing to concede the point that his magic may simply affect more than just gravity, it is a conceptual kind of magic and not strict science after all.

Mate, the concept you're looking for is inertia, because that's the practical difference between adjusting weight and mass. If you only adjust weight, your ability to change directions is still controlled by how much force you can push against your body mass. You also have most of the same issues swinging a large weapon, since you're still pushing that mass up and forward.

Pull up your force body diagrams.