What does weight % affect?

[Melandor]

I put some points into capacity in bring it below 50% but not sure if it made a difference. I could pretty easily dodge the final bosses attacks but not sure if that was because of more i frames or I'd just gotten better at dodging over the length of the demo

[Kevin Ryan]

69% is this game's medium load territory and lower than that will obviously be light.

If you endeavored to get it below 50% then it makes sense you won't ever notice the negatives of a heavy load (you're slower).

Iirc capacity also increase your legion.

[königplatzen]

Heavy load starts at either 60 or 70%, can't recall atm, but it gives a hint in the character view, that it's too heavy.
I also believe there's a light load below 30 or such, but never tried to find it, yet.

below50% does nothing special, that I'm aware of.