Hi Steven Spielberg, sorry I mean Lori
Yes, video quality can degrade - but only when you re-save with a different level of compression or another file extension. Most standard videos including TV programmes are filmed at 24 fps - that is to say, the video is made up of 24 individual, slightly different photos that are displayed and advancing each second - 24 frames per second. A standard 5min video will contain 7,200 seperate images, therefor each picture is of a lower quality than a JPG straight off a camera. If you choose to lower the saving quality to reduce the file size you will reduce the quality of each image - but only
marginally as the reduction in Mb will be spread over the entire 7,200 pictures, with each being changed only slightly - hence you not noticing. It would notice far more on an individual photograph straight from a camera as the reduction would affect the 1 single image. This is my reasoning, others may know more on the subject!
What size are your videos? Those I take are 320x240 for YouTube are run at 24fps. Of course, uploading to a video-sharing site will automatically incur their filesize-reducing software too to save on their bandwidth as far as I know - YT does this.
When you re-save to get a different filesize - are you using the same file extension (
...I like .AVI 
) and save compression? If not, this may account for it.
Hope this helps!
Take care,
Jason