I opened an existing video, cut some parts out of the video and when I save the changes the file size is bigger than the original. I was expecting it to be smaller.
I am new to video editing, what am I not understanding?
First of all, you should decide whether you can use the copy mode (which is the default in Avidemux). Obviously, if the file size gets bigger, you have chosen to re-encode. So please explain, whether it is deliberate or not.
Well if I leave in copy mode and I try to cut, I get message about "not on a key frame and will corrupt the file. "
Correct, please select cut points using up/down arrows to ensure they are on keyframes (this might be still not enough with some kinds of H.264 streams if the resulting video should be smoothly playable in VLC and other FFmpeg-based players, but this is the minimum required to avoid re-encoding).
If you absolutely need cut points to be on pictures of different type, then you have to re-encode, losing quality. In this case, it is important to know the properties of the source video to look for a viable compromise between size and quality.
Thank you, is there a resource you know where I can read about this for beginners? Or what do I need to google?
A good question. Maybe a general idea of the basics of video compression would be a proper starting point: https://en.wikipedia.org/wiki/Data_compression#Video
Regarding Avidemux usage, please just ask specific questions here and don't be afraid to explore the application. I think the official Avidemux wiki is so outdated now that it does more harm than good.