Isn't volume and weight directly proportional when you are dealing with the same substance?

I mean, all 1 gallon jugs weigh the same with the same liquid, right? So why wouldn't you be able to pack the same weight of weed into something? Density wouldn't affect it that much. It would depend on the consistency, but if you pack it tight it won't matter.

Its like, I know that a bowl full in my (now broken :|) pipe is about .3g depending on how much I fill it.