The "Project Farm" guy on youtube did some serious testing of various 18650 cells with different claimed capacities.
The big name ones (Samsung, LG, Sony, Duracell etc.) typically in the 2.5 - 3.5AH range were all reasonable, the ones with supposedly very high capacities were all fakes with high internal resistance and typically nearer 1AH actual capacity.
I have a genuine 20AH power pack. Internally that's going to be single-cell voltage with some number of parallel cells.
It's approximately 16 x 9 x 3cm over the casing; the actual battery pack is probably nearer 13 x 8 x 2cm, allowing for case thickness and the USB sockets in one end.
A genuine 12V pack with around the same AH capacity would be near three times the size!
The video is here, for reference. Go to 9:15 if you just want to see the actual capacity chart.
One interesting point he comes up with is the weight per cell - the full capacity ones all weigh 44g or more, the fakes all less than 36g.