My understanding has always been that it’s based on powers of 10.

1,000,000 vests = minnow
10,000,000 vests = dolphin
100,000,000 vests = orca
1,000,000,000 vests = whale

The stats from @steemitboard are based on these powers of 10 and I’ve seen them mentioned elsewhere too. Don’t know if it’s official from Steemit, but it does seem fairly widespread.