The critical part is not more food, but very unequal distribution of food. Quoting: "..from the time of the dinosaurs to about 3 million years ago, nutrients were evenly distributed across the ocean. But then there was a transition, in which dense aggregations of nutrients could be found along certain coastlines, while vast parts of the open ocean became virtual marine deserts."
I guess larger organisms are more likely to survive in this scenario. Maybe similar to the reasons why larger vehicles are more efficient for transportation: http://www.sustainablefreight.com.au/case-studies/bigger-is-...