There's a few different types of HDMI to AV converters out there. Some are better than others (I've even seen ones with a Zoom button).
You can sometimes on a PC playing videos in VLC get around stretch squish on the converter by playing with the aspect ratio "A" and or crop "C" keys.
If aspect ratio issues especially annoy you I made a video on a couple of solutions out there:
https://m.youtube.com/watch?v=hKjt3x4WtWU