The Yesod Ha’avodah of Slonim zy’a explains with a moshol of a powerful king who made a fancy seudah for all of his friends. After the feast, there was a lot of leftover food, which the king told his servants to distribute to the residents of his kingdom. However, even after this distribution, there still remained some leftovers. The king then said to give it to the prisoners in jail. The prisoners were taken out of their cells and seated in a nice place for their meal, as it wouldn’t be appropriate for them to eat a king’s food in their cells.
The nimshol is that Hashem has to so much compassion that He is kind even to sinners and those who committed crimes, as is stated (Tehillim 21:11): ‘And He forgives our sins because He is great.’ The Meforshim explain that Hashem’s kindness is so great that He provides even for the undeserving.
He this explains that this Shabbos is called ‘Shabbos Hagadol’ because Hashem’s kindness is aroused and He provides even for the lowest of men, as it would inappropriate for His great abundance to go to waste.