Is there top video converter in the market?

I do not know much about file converters or file dimensions for the item, however I paid attention while loading a tv show from two different sources that a file size was much smaller, the mp4, as the other was much larger, the avi. It was the same show and there wasn’t any dissimilarity in quality of image, so that I don’t know why one load was 60mb when the download from the other source was 341 MB! I only desired to find a free of charge transfering video tool what owns a big success rate of converting present avi files to mp4. video converting software needs payment, so that this is not my choice.