is there good suggestion on converting video software?

I know a little about file converting file or file measurements for that item, however I noticed while loading a television show from two different solutions that a file dimension was a lot smaller, the mp4, when the other one was a lot larger, the avi. It was the same show and there wasn’t much dissimilarity in quality of image, so that I am not familiar with why one load was 60mb when the download from the additional resource was 341 MB! I only desired to find a free transfering video tool what makes a great rate of success of transfering existing avi files to mp4. video converting software needs payment, so that this is not my choice.