While searching for memory leaking troubleshooting techniques that could be applied to 64-bit Windows (for the DHCP Server memory leak I found I had the other day), I stumbled across the answer to my problem in an internal tool (weird that I missed it from a web search the first time, but c'est la vie).
A Windows Server 2008-based DHCP server that is configured in a workgroup environment may consume too much memory
And that's my problem! One REG command (and one restart of the DHCPServer service) later, I'm waiting to see how it went, but it all looks promising, based on that article. Neat-o.