I cannot speak for
Avidemux. I use
Virtualdub to trim unwanted frames from a lossless file such as a
Huffyuv AVI. I use Direct Stream Copy option in the Video menu and do a File -> Save as AVI (F7) once I have either selected a range using Edit -> Set selection start (Home) and Set selection end (End). Alternatively, I also use those same Set selection start and end and then cut those frames from the copy loaded in
Virtualdub, then save. If a range is not selected, then it will save all the frames remaining.
There should not be anything else you have to do with Virtualdub setup to make an exact copy when in the Direct stream copy Video mode, and it should save very quickly (the rate of frames per second it processes should be very high), and not look like it is trying to re-encode the file.
However, I don't know how Virtualdub operates under the hood. I would guess it doesn't even use a codec (i.e.
Huffyuv) when it's doing a Direct stream copy. I say that, because when Direct stream copy is selected in the Video menu, the Compression option in the Video menu is disabled, so you are not able to select a destination codec.
If you are certain the edited file is still Huffyuv encoded, then my guess as to why you are seeing such a different bitrate is that this is an average over the entire video. Huffyuv is a variable rate encoder. If you look at the metrics in Virtualdub as it is encoding a capture, you'll see the rate changes depending on what is being captured. Some tapes are nice and clean and so it can encode much more efficiently, while others have a lot more noise and so the bit rate is higher because it is trying to preserve all that noise without any loss of information. If most of what you cut from the original file is just a blank screen, then the average bit rate over the 2 hour file is going to be greatly reduced because it can encode that part of the video much more efficiently than the few minutes of video you are trimming out of the 2 hour file. If it can encode almost 2 hours of blank screen at a very low bitrate, but the few minutes of video you are saving is encoded at a bitrate several times higher than that, then the average of the 2 hours of blank + your few minutes of video is going to be much lower than when you only save the few minutes of video.
Hope this helps.
