The Songhai empire expanded its wealth and power by:

Songhai rulers subsequently took advantage of the weakened Mali Empire to expand Songhai rule. Under the rule of Sonni Ali, the Songhai surpassed the Malian Empire in area, wealth, and power, absorbing vast areas of the Mali Empire and reached its greatest extent.

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The way this small state - large state compromise guards against tyranny is that small states and large states have one note: i am sorry but i was not able to give you the full answer. but i got this answer from