lol.. ok, vdub isn't that intuitive that it saves avis as you want to have them.
. if you don't set compression methods, it stores the result as uncompressed (raw) avi, and this needs some space
before you save the final avi, go into video compression settings and chose a compression method. for a first test try "divx low motion" (you also can configure each compressor - if it's necessary you can set a different compression data rate here, but don't set it higher than the original is. you can check the original data rate via file menu->file information after you've loaded the original). audio settings usually are ok, but sometimes it's necessary to set a compression method here, too (you can see if it's necessary when the encoding process is running - if the audio track tooks far too much (>10 % of estimated size), then go to audio settings, chose full processing mode and a compression, f.e. mpg layer 3. 6kb/s is enough).
in vdub the settings aren't stored with the software when you finish the program, so each time you use it, you have to make the compression settings first. you can do that any time before you save a result. but you can save and load your settings manually (save/load processing settings in file menu)