The phorid genus Megaselia Rondani is possibly one of the biggest genera of insects. I say “possibly” because we still don’t know if we should consider all the species currently classified as Megaselia as belonging to a single group. Recently molecular work by Sibylle Häggqvist seems to indicate that most of Megaselia is monophyletic (forms a natural group), but much more work needs to be done.
Just for fun, I made a graph of the described species of phorids, showing the number of species per genus. This is a representation of how big Megaselia is within the Phoridae. Note that this is the number of DESCRIBED species, and does not take into account all the unknowns. 
For the phorid researchers: does this treemap surprise you? It did me, a little. I thought Megaselia would be even more dominant. Presumably, once we get to do all the world’s fauna, Megaselia will end up being closer to half of the family.
Or maybe not.
Yes. It does surprise me a little, as well. “Megaselia make up almost half of all phorids” is a line that is always in the back of my mind. Is that presumption skewing the taxonomy? I don’t know.
Visually, the non-Megaselia section of the map gets a bit more area than Megaselia just due to the amount of extra border space, but that probably isn’t significant.
I think most taxonomists outside Phoridae, would think it is strange to have a genus that subsumes 30-40% of the species. In almost all metazoan families, genera that subsume over 10% of the species are almost unheard-of.
Thanks John. It would be interesting to see similar tree maps for other families of Diptera.