Can I control this?

Lately the two main video sources that have been coming into my studio are DVCPRO-HD 720p -- tapeless recording on Firestore usually; and HDV (1080i 29.97). I don't want to edit in HDV so I have been capturing the HDV through my Kona 3, converting it to 720p 59.94. (I capture by playing the HDV tape in a Sony deck and running the component out through an AJA HD 10AVA and then into the Kona 3.) Editing is done in FCP on an Octocore.

Here's my problem: The HDV-sourced files are MUCH larger than the native 720p files. So they take up a lot more drive space, exports are much slower, etc. And at no gain in quality. Example: a native DVCPRO 720p 5-second clip is about 35 megs. A 720p 5-second clip created by Kona 3 from HDV source is 95 megs. Almost triple the size.

Why is that? Is there anything I can do to bring down that data rate/file size? I feel like I'm wasting a lot of time and space.