If you shoot a Molot or a Saiga Shotgun I am sure you are annoyingly very familiar with how often you have to change magazines. The normal one only holds 8 or 10 rounds, which can fly out in matters of seconds. The magazine is empty before you read this sentence. With the Powermag it takes a little longer.
Maxrounds from Czech Republic (great name!) did a size comparison between their PowerMag 650 and the Magpul and the result we can see here.
One magazine holds 20 rounds of 12 gauge, the other 60 rounds of 223 Remington (or 300 Blackout).
In fact, when competing in IPSC Shotgun there is a 10 round limit in Open, so when and how fast you can do your magazine changes is a big part of the game.
Top picture: PowerMag for Molot Vepr 12 vs. Magpul D-60 vs. PowerMag for Saiga 12.
Below: PowerMag for Molot Vepr 12
Below: Magpul D-60 vs. PowerMag for Saiga 12
According to Maxrounds their PowerMag is the smallest and lightest 12 ga 20 rounds drum magazine in the World.