Hard to tell by your question, but as a rule subsequent encoding nearly always reduce quality. That said, all video files are encoded using some form of compression (codec), and often it's not done very efficiently. For example, when creating short video using Camtasia, they output file can be several hundred MBs. Running it through Handbrake can reduce this to 10s of MBs with no readily apparent loss in visual quality. There are many factors in this - the main thing is to try it and see if it works for you.
