I don't have the exact measurements, but I own both the vavin gm and piano--I've looked at the vavin pm.
The vavin pm is the smallest by far--it's very small. Next is the cabas piano. It might actually hold more than the vavin because it's softer and you can stuff it. However, if you fill it more "normally" it's smaller than the vavin gm.
Finally, the vavin gm. I love this bag! It doesn't seem big at all when carried, but it holds a ton. It's tall and has this cool organizer structure inside, with four large pockets built in (one on each side). Then, the middle part if open and it can hold quite a bit...