How do you change the RAM used on your server?

When starting my server I type:

java -Xmx1024M -Xms1024M -jar minecraft_server.jar nogui

I'm assuming everybody does that but just in case I am just showing for reference. What I am trying to figure out is how to use 2048M of Ram instead of 1gb. When I type 2048 instead of the 1024 it still shows that its just using 1gb. how do i change that?

edit: i use a mac