If only the eleven largest states joined this pact, the required 270 electoral votes would be in their control.
You're also taking away voting power from other states.
Not exactly. Rather, you're ceding it to the nation as a whole, because the result in those top 11 states depends on the result in the national popular vote. It's not as if the national popular vote is some disembodied entity that exists external to any statewide result.
Also, the required 270 votes are already in the control of the top 11 states.