If you can simultaneously “explain” why humpack whales, Megaptera novaeangliae, are characterized by extremely long pectoral flippers, carry armament in the form of barnacle patches on their chins and on tuberosities on this pectoral flippers, and have disproptionately thick blubber compared with balaenopters, you will get the answer.

As encouragement, be assured that the resultant equation of constraint is more than interesting.

Current estimates put the split of blue whales at ~15 +/- 3 Ma and the split of Baleen whales at ~23 +/- 2.5 Ma (Sasaki et al. 2005). Either 15 or 5 Ma is a pretty long time for a increase in size to occur? There are multiple studies showing rapid changes in size over mere decades (Huey and Gilchrist) to thousands of years (Smith et al. 1995). Granted the examples come from species much smaller but there is no indication that it is easier for a small organism to increase its size compared to a large one. There is also little evidence of major shifts in size in the last 20 million years (http://www.nceas.ucsb.edu/~alroy/Paleocene.html). Nonetheless a ripe area of research with many outstanding questions.

Also consider that whales are simply one example, but there is evidence from multiple phyla that morphological extremes are reached very early in a clades history, often indicating constraints on form.

@ Stella
Indeed oxygen is important for the size of life see the paper I coauthored in PNAS with colleagues at http://www.pnas.org/content/106/1/24.abstract that provides a long term and broad taxonomic perspective.