In whale brain at least, it appears the larger size is more related to extra glial cells and other factors:

My pet theory on this is that glial cells are known to stimulate synapse growth and they support synapse function (e.g. by clearing up after firing) - and so the enormous quantity of glial cells in whale brains (9 times as many glial cells in the sperm whale than in the human) - and their huge neurons - both point to an astronomical number of synapses.

"Glia Cells Help Neurons Build Synapses"

• http://www.scientificamerican.com/article.cfm?id=glia-cells-help-neurons-b

Evidence from actual synapse counts in dolphin brains bears on this issue too.