[inglor_cz]

In a pre-industrial economy, size of your economy was dependent on a) having a lot of peasants, b) having a lot of reasonably fertile soil.

China and India between them counted for at least half of humanity during that period, and probably more. No wonder they were the largest economies.

[achow]

You are forgetting..

c) Having lots of ships, sailors and powerful army (many in Europe)

d) Being a trading hub (Italy)

e) Having access to lots of valuable metals, salt, gold (richest person in history was Mansa Musa, King of Mali)

[inglor_cz]

Statistical outliers like very rich kings or ports do attract attention, but there was no way to avoid low productivity of agriculture overall. At least 80 per cent of the population had to tend the crops, which lowered the average GDP of entire regions / countries.