I certainly agree there's a rich-get-richer phenomena, but I'm not sure there's evidence that mid-size VCs are disappearing.
The generic phenomena is "preferential attachment", which creates a power-law distribution (nearly indistinguishable from log-normal). It's common for observers to believe there's two categories of size, either big or small. This is the origin of "tipping point" theories. However, in a power-law or log-normal, there is no discontinuity. Observers are distracted by the enormous difference between the biggest and next-biggest that they don't notice the same relationship continues all the way down to the smallest.
The generic phenomena is "preferential attachment", which creates a power-law distribution (nearly indistinguishable from log-normal). It's common for observers to believe there's two categories of size, either big or small. This is the origin of "tipping point" theories. However, in a power-law or log-normal, there is no discontinuity. Observers are distracted by the enormous difference between the biggest and next-biggest that they don't notice the same relationship continues all the way down to the smallest.