There is no point in checking the whole movie file for malware - as the malware cannot be hiding “in the middle”.
So, for known file types, only the relevant parts of the files (those that can be affected by malware) are scanned - scanning anything else would be useless.

It’s got nothing to do with the right-click scan specifically, it’s a property of the scanning engine (and the areas to scan are “dynamic” - meaning that if a malware appears tomorrow which would be hiding somewhere else and the corresponding detection is added, avast! starts to scan that “somewhere else” at the same time).