If for example I have two video files, both of similar characteristics, file type, encoding, resolution, etc and starting at the same point but A goes on for 10 seconds while B goes on for 20. If A's file size is 10MB and B's is 20MB, if I read in e.g. the first 5MB from both will the major video encoding formats' binary sequences match for that 5MB?
E.G. MP4, AVI, MOV, WMV?
No, different containers work differently, the first X bytes will not contain the same number of frames. In some cases like mp4, you may get audio, or metadata, and no video at all, or you may get bytes that can not be interpreted without information that comes later in the file.