why is hs's 1080p s of the lowest size ? is it because the quality is low or is it of better encoding?

@ashfaq The encoding is the same, as they all use H.264 (MPEG-4 AVC). You can say the quality is lower than others, since it has a lower bit rate. Though the resolution, frame rate and coding format are the same, a lower bit rate means a larger loss ratio compared to the original video. The advantage is that the size of compressed file could be much smaller (as you can see, 541.6 MiB compared to 800+ MiB).

@maristie thanks so in a nutshell, to make it in low size while retaining maximum quality possible they do this?

@ashfaq Sorry for my late reply. Exactly.