Goldbogen and his colleagues found that big fin whales are not just scaled-up versions of little fin whales. Instead, as their bodies get bigger, their mouths get much bigger. Small fin whales can swallow up about 90% of their own body weight. Very big ones can gulp 160%. In other words, big fin whales need more and more energy to handle the bigger slugs of water they gulp. As their body increases in size, the energy their bodies demand rises faster than the extra energy they can get from their food.
This scaling may explain some of the weird diving patterns found in lunge-feeding whales. Blue whales are twice as big as humpback whales, for example, but both species dive for the same period of time (about eight minutes) and to the same depth (148 meters). All things being equal, you’d expect that blue whales would be able to dive deeper and longer, because they could store more oxygen in their bigger bodies. Blue whales also make fewer lunges than humpback whales (6 versus 15). It’s possible that the gigantic blue whales are hard up against a size limit. They need so much energy for their lunges that they can’t afford to hold their breath longer, and they can only manage to make a few lunges before they run out of reserves and have to head for the surface.