HEVC gets better quality at smaller file sizes, so probably that.
Correct is: HEVC has the same quality as AVC at approx 40% smaller file size. Disadvantage: encoding to HEVC takes much longer than encoding to AVC. So probably the time you save with uploading a smaller file will not weight against the time you lose while encoding.
@3POINT and @Musicvid +1 I compared both, AVC and HEVC renders. Rendertime for HEVC is at least 10% longer on my laptop and the filesize without visible loss ( for me 😉) is between 25-50% smaller. Only for users without enough space that is maybe important but it can not bother me compared to the old DV and Mpeg2 times.