I might be wrong but it seems to me that the ".mov" is bigger than the equivalent ".mov" and I wonder what are the differences. Is the "mov" better than "m4v"?
M4V, MP4, and MOV are simply containers. The same compressed H.264/AAC data can be contained in each.
M4V - The M4V file container is used exclusively for H.264/AAC content either protected or unprotected. Limited use of other data tracks can also be found in this container such as "Chapter" or "alternate audio" tracks. This container can also be associated with external graphic data as normally seen in iTunes when "Artwork" is added. By default, this container normally opens in iTunes when double-clicked.
MP4 - The MP4 file is much like the MP4 file container but may additionally hold MPEG4/AAC data as the primary video/audio compression format. It too may contain supplemental data tracks or be associated with external "Artwork" but, by default, opens in the QT player.
MOV - The MOV file container is a generic file container which may contain up to 99 tracks of any valid audio, video, text, "Chapter", "Tween", ""Timecode", "sprite", etc. form of data. It cannot be associated with external "Artwork" but, like M4V and MP4, can have a designated "poster" frame. Any valid file saved using the "Save As..." File option is automatically placed in the MOV file container by default regardless of its original compression format and file container type.
While there are "internal" differences, all three containers have the same basic purpose. It is likely these differences which you see when comparing "file size" even when the actual compressed audio and video data is exactly the same within the container.