If your law department wants to obtain a fixed-price bid for a large block of work, it must needs deal with large law firms. Those firms with, say, 800-plus lawyers can take on an entire portfolio of work. Massive firms can absorb more financial risk, they can deploy the specialist and grunt talent as needed, they can systemize their procedures, and they can invest in software and other resources. Increasingly, as more law firms pass the thousand-lawyer mark, these gargantuan ones can risk relatively Lilliputian bundles of corporate work.
The tradeoff law departments will have to make is between a higher cost structure and more certainty for the budget.