yes, i found many similar topics of rising capacity, but i have some other proposal:
why don't make capacity dependent from the quality of waterskin/flask without changing base capacity?
i offer make it equal base_capacity * root(quality / 10) of third (or even forth) degree
so only q80(q160) waterskin/flask will have 4L/6L
maybe also lower base capacity, so that 4 flasks or 2 skins will contain more than a bucket only with really high quality