In order to edit the highly compressed 15-GOP MPEG-2 format that makes up HDV, some editing solutions, upon capture, transcode the video to a more friendly and robust editing codec. These "digital intermediate" files are always significantly larger than HDV in order to maintain quality, while the codecs themselves are prided on being "visually lossless."
IIRC the lower end of Apple products (iMovie?) uses an Apple intermediate format. Premiere 1.5.1 uses an older version of the Cineform codec, and I personally use Cineform's AspectHD in combination with Premiere Pro. I'm not entirely sure about the Sony Vegas solution, although it may use proxies.
The new version of FCP claims "native" HDV editing - which may or may not be a good thing depending on where your cuts are.
Supposing you are talking about storing the HDV Stream it takes up as much space as DV. Intermediate codecs are always different. If you are using a Mac and FCP 5 don't use the Apple Intermediate Codec. FCP can quite well deal with HDV. It's no matter where your cuts are or what filters you use.