The Mishna (9b) discusses under what circumstances a man will be held responsible for damage that he causes. One of the rulings is that a person who is responsible for only part of an obstacle that causes damage (e.g. someone who completes a hole in the public thoroughfare that otherwise would not have caused significant damage) will nevertheless be held liable for the entire damage that took place.
In discussing this case, the Gemara brings a baraita that presents the following situation – five people were sitting on a bench without causing it any damage, but when an additional person joined them, the bench broke. According to the baraita, the last person is seen as having caused the damage, and he will be held liable to pay restitution. Rav Pappa explains that this ruling will be true only in situations where it is someone like Papa bar Aba – who was known as a particularly heavy person – who will be seen as being responsible for the damage.
Many of the commentaries ask why the Gemara needs to introduce a heavy set person into this story – shouldn’t anyone whose weight ultimately breaks the bench be held responsible?
The Rashbam explains that a bench is made available for use under normal circumstances. Since an average bench is not made for someone like Papa bar Aba, by sitting on it he is misusing the bench and will be held responsible for its having broken.
The Maharshal in his Yam shel Shlomo suggests that ordinarily a bench does not break in an instant – there are first creaking noises and other indications that it is about to break, and the people sitting on it usually have the opportunity to get off of it in time to keep it from breaking. Once Papa bar Aba sat down, his weight broke the bench immediately, which is why he was held responsible.
According to the Sefer ha-Eshkol the Gemara was simply bringing a case where is would be clear to everyone that it was the additional weight of the last person that made the bench break. The Meiri suggests that Papa bar Aba was invoked mainly to draw a vivid picture of the incident.