I think it is somewhat silly to do so. People have an easy way around it- refills, more drinks, etc.
However I do think it is a step in the right direction. People have a different idea over what precicely a “serving size” is. Changing perception on what a full meal should look like will greatly help people.
Besides, most already lack the motivation to help themselves. Forcing them to help themselvse is the best thing others can do.