Use another freeware tool called Gspot or look in the information window of virtualdub to see what codec was used to compress the original file and what bitrate was used.
When you save the file in virtualdub you will have to use full processing mode and under compression choose the codec to compress with. If it's divx or xvid for example then choose the bitrate that is close to what the original file was, then use "save as" to save the new file. Processing will be a bit slower because with filters you need to reprocess the file completely but you can view the changed frames in the output window and you can stop the process if your not satisfied.
btw) If you stop the process whatever was done is not deleted. This lets you look at the results before doing a whole file for nothing. Also if you only want to do part of the video in the middle, use the mark-in and mark-out feature in the main display window and then apply your filters and save as usual. Only the highllighted portion will be processed and saved. Again this allows you to do a much quicker test on a selected portion of the video.