If you use MP4, your audio is going to be AAC, usually 2.0. Sometimes 5.1, but no AV receiver is AAC-compatible.
If you don't have an AV receiver, why bother with 5.1 DTS and AC3 sound? Ok na sa 'yo ang AAC 2.0. Hindi ka naman pala maselan, e di liitan na rin natin ang video size. That's why you have the 720p 700MB MP4 with AAC 2.0 audio.
But if you do have an AV receiver, you're going to want AC3 or DTS multichannel sound. MKV dapat ang container. Kung maselan ka sa sound, chances are, maselan ka rin sa video quality. Taasan na rin natin ang video bit rate. Ang effect, lalaki ang file size ng video. But your TV will be happy with the video, and your AV receiver will be happy with the audio. That's why 4.5GB to 16GB MKV files with AC3 or DTS multicahnnel audio are popular. Because quality audio and quality video go together.
At iyan ang alamat kung bakit traditionally maliit ang MP4 at malaki ang MKV.
May AC3 compatibility na raw ngayon ang MP4? Too late na, nasanay na ang users sa MKV e.