The Start and Stop button are of identical size and prominence.
The office microwave has a button called Quick Minute. It adds one minute to the time but does not start the cycle-- you have to hit Quick Minute - Quick Minute - Start if you want a two-minute cycle at full power. The Stop and Start buttons are the same size, more or less, but the Stop button is graphically prominent-- I think it's outlined.
I find the UI on the one I have at home to be much more useful than the one at the office. It requires less button-pushing for the tasks I use most often. Plus, I'm constantly hitting Stop rather than Start at work.
I'm a huge fan of optimizing for the things that people do most frequently. For my usage patterns, at least, my home oven has a far superior design. It looks like most people agreed with me on both counts.