If it was wider it would fall off or spin on the end of the crankshaft. It's the tight fit that holds it on there, not the bolt on the front.
This is, of course, assuming you followed the directions with the balancer and ensured it's dimensions fit with your crankshaft. Remember, it's designed to have a specific tool to do this, things can all be as easy as a "tool-less" computer case.
To keep the crankshaft from moving you have to put the transmission in gear and block the wheels, assuming a 5 speed. With an automatic you pull the inspection plate and place a bar into a slot in the flexplate so that it prevents the rotation.