Welcome to the forums.
No, you aren't missing anything. HDV and DVCAM have the same file size. This is because HDV is HIGHLY compressed. Both formats are recorded on the same sort of tape...miniDV sized. And since tapes capture footage digitally, as data, it stands to reason that they have the same data rate...and they do. So capturing HDv as DV gets you nothing...just capture HDV native.
Get more drives. You can capture your footage at a lower quality (offline RT) and do the offline/online workflow, but the potential headaches that can cause are well worth avoiding, if you can. Drives are cheap.
Drawback to editing HDV native? That format is processor intensive...not easy to work with. Many people opt to capture it as DVCPRO HD (requires a capture card), but that also increases file sizes. Rendering and conforming HDV takes a long time.
Again, OFFLINE RT is a low res option for capturing footage. Drawback is that you cannot view this on an external production monitor. And Onlining (racapturing at full res) with FCP isn't perfect...requires adjustments. I still recommend more drives.
Shane