Chainmaille only counts as heavy if it is iron or steel. This is usually (a) more expensive and (b) wieghs a lot more, than, maille incorporating aluminium, neoprene, or similar materials.
I suspect it could be argued that the damage distribution of chain and the layers worn under it are pretty useful against arrows, but then the same could be said for cuibolli.
I choose to look at [the classification of steel chain as heavy armour] as a balancing point in the Armour Debate. (see below)
Player 1) “I have this awesome steel chainmaille, it looks great, and adds extra flavour and immersion to your game!”
Ref) “That’s great, but it’s not heavy armour, and won’t stop arrows.”
Player 2) “I’ve made this armour out of gaffa tape and foam, it looks like plate armour, wieghs less than a kg, and counts as heavy!”
- “So I invested considerable cash in buying kit that looks great, but is heavier and less mechanically good than his gaffa-plate?!”
- “Chainmaille is expensive! Are you going to penalise me for being unable to afford good armour physreps?”
- “I want to play my character concept with immersion and a touch of historical accuracy!”
- “I want to play my character concept without dumping £200 on one piece of costume!”
Ref) “Okay, here’s what we’ll do. 1, given that it costs a lot and wieghs more, the chain is heavy armour. That’s the penalty and the downside. 2, not the gaffa-plate. Have you considered this resin armour? It looks good, is fairly cheap, and is much more durable, so it’ll have resale value…”
- “What if I knitted a chainmaille hauberk…I could make it look really good?”
Ref) As the weight and the cost are the balance factors… no."
(this is entirely from my head, and may be as accurate as knit-maille)