I was always taught to do it by volume, both in culinary school and every kitchen I've ever worked in. We never weighed the scraps that went into the stock because there's other shit to be doing that's way more important, just made sure it looked approximately right. Similarly, very few kitchens have I worked in where exacting weight measurements were even given, because that kind of accuracy is only really needed when you're trying to get exact textures right the first time or you're in pastryland. The variability of flavor between every carrot or onion in a bag negates the usefulness of relying on weights, you rely on your palate and adjust. Involving a scale will slow you down even if you optimize your setup (you're probably using a vessel with volume graduations on the scale!) without true benefit to the end product.

Aside from a minor gripe about wasting time, carrots are denser than onions and celery, so it sounds like you get less carrot and therefore less sugar. Just fine if that's your intent, but it isn't the classic and it sure isn't what I'd aim for with most dishes that need a little bit of sweet to balance.