I hate this trope of ''don't wear armour cause it slows you down'', it is so stupid and just points to a lack of knowledge.
A full set of iron armour complete with padding and all only weights about 24 kilogram and it is evenly distributed across the body. We wear backpacks heavier than that when we go hiking or if you go on a particurarly big shopping spree, carry bags heavier than that.
People in full armour can run unimpeded, ran up ladders, do somersaults and many other acrobatic feats without issue.
Her tits are nice tho.
I just brain off and say this manga's universe has defense be low priority, because attack power is so high across the board.
That said, if Eugene only gets the iron gauntlet (which seems to be the focus), it would cause an imbalance. And for a weighted gauntlet in particular, that would affect his attack and defense capabilities (since i assume in this fantasy setting, the sword will also be able to block things better than a shield).
She did say to get something light and durable, not no armor at all.
I do agree for what you say in particular though, even if by modern standards an extra 50+ pounds is pretty rough to lug around unless you're trained/experienced.
This is also completely ignoring the very first chapter's opening pages, where she's in literal full armor. Probably not made of metal or iron, and perhaps some other fantasy composite. Maybe demon hide or something.