avconv/ffmpeg temp files?

Ubuntu 14.04

I'm in the process of converting movies from .MKV to .MP4. I convert some, then once completed, remove the .mkv file, leaving the .mp4. As the .mp4 file is slightly smaller than the .MKV, my free space should be the same, or slightly more, however, I'm losing space i this process. for every 10GB's of files I convert, I lose 10GB's of space. So my question is....does avconv/ffmeg create temp files while processing movies, and if so, how can I find these to clean them up and reclaim my disk space?

thanks