Editing various video formats

I'm looking for some simple utility to cu and reduce the size of some
videoclips, they are .mpg, .avi, .rm and so on. Now when I try to edit them
(mainly to cut unuseful pieces) by means of Cyberlink Power Directory (the
sw I got with my JVC Everio videocamera) I'm puzzled since I'm asked
questions such as "which type of mpeg? which quality (good, best)". After I
choose some value just to try, production of the video starts and can last
several minutes, so I think it is rearranging or even modifying the original
file. Indeed at the end the size of the retailed file often does not match
with the expected one. This happens also with other programs I found on the
Internet.
Now I only want to cut my videoclip in the simplest way, namely to leave
unchanged its parameters - it only has to take some selected pieces and
record them in the output file by leaving them unchanged, I think the only
operation would be to stitch them together and to modify something in the
header
Does such minimal sw exist?
il barbi