TL;DR: You can't.
The file you currently have is probably encoded with an MPEG-4 Part II encoder such as XviD or DivX. Those are the ones being used mostly for distribution over internet, etc., because they retain acceptable quality at CD-R file sizes.
Why you're getting big files:
DVDs however use an older codec, namely MPEG-2. Remember that a DVD has much more space to offer, and the standard is a bit older. The main difference between those two codecs is that MPEG-4 is much more efficient in terms of size, in contrast to MPEG-2, which will take much more space than an MPEG-4 video at the same quality.
So whenever you take an MPEG-4 video and convert it into a video suitable for a DVD, you will always get a bigger file.
Why the quality isn't that good:
Also, whenever you take an already encoded video (for example in MPEG-4) and convert it into another compressed video (for example to MPEG-2), you are going to lose quality no matter what. How much of that quality loss is perceivable is another question. It shouldn't be that much in your case. Also, don't forget that a DVD video has a defined maximum size for the video frames, for example 720 × 576. If your video was bigger than that before, it will be downscaled, which means another loss of quality.
What options do you have?
If you want to watch this movie on a DVD player, your best bet would be to find a player that accepts XviD or DivX files (assuming your original file is one). Most of them do, nowadays. So you could just burn the original movie to the DVD without needing to convert it before. That way you will not lose any quality and the movie will still take up the same space.