Speaking from direct experience, large, reputable

Speaking from direct experience, large, reputable firms do not take on small clients without there being significant reason. Small clients with small issues are farmed out to "friends" (other firms). Large, reputable firms take small clients on retainer for the potential for large wins (only after doing quiet DD on said small client)... the exception to this is if an existing large, reputable client brings the small client through the door.

But you know this already....