Unions haven't gotten too big and powerful. They were MUCH bigger and MUCH more powerful 50 or 75 years ago.
They've just outlived their usefulness in most fields.
Unions worked before technology made it possible to get almost anyone, anywhere in the world to do the same job. It's impossible for labor to bargain when they have no bargaining power.
Except for the NFL Players Union. They can still strike and actually shut things down.
