If you are capturing - or more accurately transferring - using Firewire, then DV is your only option in real time. This method transfers the DV tapes, with no loss of quality at all. But as you have discovered, the files need about 13GB of storage, per hour of video.
These files have several advantages though.
1) They maintain the full quality of the original
2) They are easy to edit
3) You can use high quality settings to make good mp4 copies, with freeware like Streamclip, for example.
The other thing is that these large DV files are only likely to be temporary. You would presuambly delete them once you've got your mp4 copies?
If you decide to 'capture' instead as MPG2, as Graham has suggested, you will not be using the Firewire port to do this, but rather the analogue (composite) output from the camcorder, via some other sort of capture tool.
A DVD recorder, or mpg2 hardware, presumably?
This process will involve losing some quality, although hopefully not too much These MPG2 files will also not be quite so easy to edit, if you need to.
You will still have to convert them to mp4 anyway.
AFAIK, there is no way of transferring DV directly to mp4, in real time......