Hi, I'm totaly new with video editing.
I'm tryig to screen capture something and elaborate it later.
I've been running a simple test: I recorded a short video by recordmydesktop (.ogv) with no audio.
I imported it as a new clip and exported the project in 5 formats.
These are the results:

original.ogv 370K
test.ogv 1,2M
test.avi (xvid) 1,2M
test.mp4 1,3M
test.mpg 1,5M
test.webm 103K

The screen capture is 640x480 30fps and the video project is video4linux capture (same size and fps).
What surprises me is that, exporting in the same format (.ogv) the size increases so much without adding or changing anything.
May you explain me the reason?

I used kdenlive 0.92 on debian wheezy.

First, you are on a totally outdated and buggy kdenlive version. 0.9.8 hast just been released, you're on 0.9.2. But this doesn't really is about output file sizes.

The reason for different video file sizes is clear and can also be found in the docs: Kdenlive is a nonlinear video editor that renders the output anew. Kdenlive isn't simply piping the input video through but instead composes each frame individually and let the output codec then do its work. Different sizes come from these codecs and their settings. Different codecs work differently.

And in case you now want to ask this question: no, Kdenlive cannot simply pipe through video, use ffmpeg for such tasks and live with the situation that you can only cut at I frames.

In addition to what TheDiveO wrote: A sample clip of 370kb can easily lead to erroneous conclusions, as the different header sizes of different codecs are significant, while they will have no measurable effect when talking about 1GB video files.