If we had more data on the sizes of law firms retained by U.S. law departments, the industry would have some guidelines for typical distributions. For example, it might be a rule of thumb that roughly one-third of the law firms retained by a typical U.S. law department would be small, say with less than 10 lawyers. Perhaps one-third of the firms would be in the intermediate category of 11-to-20 lawyers. The final third would be large firms with more than 21 lawyers.
The guidelines would vary depending on how you rank the firms. If it is by total fees paid perhaps larger firms of predominate. For example, 60% of fees to the top third of firms by size. If you rank by number of matters handled that could present a very different picture. This data from a representative group of law departments would likely undermine claims about convergence.