We know that Iron Islands has a huge navy. It is their main strength. We also know that Iron Islands are barren land, very small and even if they fish extensively, it should not support a population as great as the North. In fact, it was one of their motives when they waged war against the North, to obtain Lebensraum for their people.
I am not really shocked with their ship building efficiency. They might steal the wood easily from many unprotected forests in the North or in the Riverlands. The Iron Fleet should already have around 500 ships in total. Building 500 more in one or two years for a seafarer nation should not be the greatest challenge.
However, for such a small nation to man even 500 ships efficiently puzzles me. In a world where the North can muster 20000 men (if they took their time it would at most be 40000), how did Iron Islands man 1000 ships? What would be the crew per ship?
One of the best comparisons I could find is the taking of Deepwood Motte, where Asha Greyjoy commaned 30 longships and about 1000 men, making it about 30 men per ship. Battle for Deepwood Motte is another possible reference point, where it is claimed that Asha Greyjoy had 4 longships and 200 men. This makes 50 men per longship.
So, how can Iron Islands support a fighting population of 30000 while the best North can do is 40000?
Book answers are preferred but TV show answers are also welcome.