I've had some bum water carriers in my time, so to be honest, when I ordered this I had very low expectations. When it arrived, it's 13lt label seemed a little bit optimistic as it looked very thin indeed.
However, when I came to fill it up, it expanded at an incredible rate, and to start off it didn't seem like it was filling much at all.
I think that the 13lt will refer to if you filled it to it's maximum capacity which will be some what impractical. It does however hold a good amount of water and if you don't have the space to lump huge gerry cans around with you, it packs down nice and flat, but is also durable.