I may not be following you correctly and my suggestion may not be 100% correct since we are talking several possibilities here.
But I'll attempt to make sense of this.
It depends on the program that was used to make the video and to which size the program used to create the file, when videos are converted the program that converts the video may remove un-needed frames.
Lets briefly mention DVDs that are dual layered, those use up more space than what an average DVD can support, some being as large as 8.0 GB which an average 4.7 Gb DVD can not store that data, so the process is to SHRINK the content so it fits the 4.7 Gb media.
I realize you are referring to files on a computer and not a DVD movie, but the point I am making is some conversion programs down-size the file as it is being converted to remove un-necessary frames.
My next question is do these videos have any content removed from them in the conversion process? In other words, are any scenes missing in the converted version vs. the origional?
If not then what your conversion program did was it down-sized the size of each frame so it consumes less space.
In a few cases, this can affect the quality of the video frames to a less desirable quality, if this is the case, you should go to the properties of your program and change the conversion settings, so it does not sacrifice quality over size.
Edited by MrBruce1959, 09 August 2010 - 01:20 PM.