Why did whales end up larger than most sauropods and what is a whale's limits?
As best we can tell food availability is the relevant limits in whales.  its true that the bigger an animal gets the more energy efficient it is, it needs less food proportionally but still needs more total amounts of food. There is a limit to how much food you can find and eat, especially in the lean parts of the year. This is actually why we see dwarfism on islands lower food availability favors smaller size (less overall food needs). Finding food and even feeding itself takes energy and it reaches a point where the energy you expend is equal to or less than the energy you can extract from your food.
There are other limits like blood pressure, internal surface area, digestion time, heat buildup, but whales have not reached any of these.
https://science.sciencemag.org/content/366/6471/1367