In the US, it is very uncommon for MBB to serve companies with less than $1B in revenue. Exception being PE owned companies looking for a turnaround.
This is mostly because of the high fees (7 to 8 figures), but also because these companies believe in providing 10x impact. I have seen a McKinsey Senior Partner reject an engagement proposal from a medium scale company because he believed we could not provide more than twice the impact of our fees. Very inspirational!