Require good video converter?

I do not know much about file converting file or file dimensions for that matter, however I paid attention while downloading a tv show from two different sources that a file size was a lot smaller, the mp4, while the other one was much bigger, the avi. It was the equal display and there wasn’t much unlikeness in image quality, so that I don’t know the fator one download was 60mb when the download from the other source was 341 MB! I only wanted to find a free of charge video converting tool what owns a great success rate of transfering existing avi documents to mp4. best video converter costs, so it is not my choice.