The Smith machine does the job of balancing the bar for you. All that you need to do is push it up.

Therefore, you normally find that you can add more plates to the Smith machine when you perform a certain exercise because there is no balancing involved. I've stopped using the Smith machine for that very reason.

Try doing the regular bench press with the standard-size Olympic bar. If it's your first time, you might have difficulties pushing the bar up straight.