Completely different scenarios. Stealing an office pen is slightly immoral but basically affects no one. Stealing a lunch ruins someone's day, and is a direct insult to their time and existence. An absolute false equivalence and you know it.

Sure, taking someone's lunch is worse than stealing a pen, but firing them is overkill. The punishment does not fit the crime.

If you believe that theft is an isolated incident then sure. I'd argue that someone who steals a coworker's lunch is not a good person in other areas of their life.