Several people have pointed out that I might be mistaking multiple copies of my iTunes library in Time Machine with hardlinks, which is the Unix magic Time Machine uses to keep references to unchanged data in its backup. In this case, however, I don't think I am.
A hardlink to a file doesn't take any additional space. When I checked the amount of space consumed on my backup drive two days ago, it reported 92GB had been filled. Then, this morning when I woke the iMac up, Time Machine started doing its thing. It was running for longer than normal, so I opened the preference to see what it was doing and it reported it was copying ~50GB to disk. When I used FileMerge to compare the last two backups, the second had a full copy of the library—not hardlinks only—and the space consumed had changed to 142GB.
As far as I understand it, creating hardlinks does not require copying data to disk, and it wouldn't report more space as consumed. So in this case I believe I'm being affected by strange backup behavior, and not a misunderstanding of the underlying technology. Or, possibly, Finder is lying to me about free space.