They don't have nearly the same power on a federal level. But the people of Kansas do have some power, thanks to the Senate. New York has much more power in the House than Kansas.

Let me turn the questions around: how would it be fair for Kansas to have no power federally at all?

New York can pass laws about what happens in New York. But if New York wants to pass laws about what happens in Kansas, the people of Kansas deserve a voice.

States are independent political entities representing very different groups of people. The EU does the exact same thing as the US federal government. If you don't give smaller states some amount of power and respect, there is not reason for them to participate on your federation.