Indicators of island complexity and all loading screen messages

The most non-trivial loading screen message: "Islands with fewer enemy types have fewer enemies overall". Also, I noticed that average size of the islands increases over time.

So it seems to be 3 main indicators of the island complexity: 1. Islands with more enemy types has higher complexity. 2. Bigger islands - higher complexity. 3. The further the island is to the right, the more difficult it is.