I've had similar 'troubles' with the volcano, where a small to moderate dose of freebase DMT will fill a bag 2 or more times, it seems like there's always a bit left.

It's the one of the best ways to stretch out vaping pure freebase DMT but for those wanting a high dose in a few lungfulls it can a bit too tricky, the only option really is loading more in so more gets into the bag but then that means your dose will be a best guess.

I love the GVG but personally like having the vapor in a volcano bag as i can see how thick the vapor is/how much i'm taking in.

I generally prefer riding the DMT wave, then going a bit harder when i feel psyched for it.