Upper limit on file size of video pano -> mobile "video_auto_start"

I've been working with "auto starting" 360 videos for mobile.
It seems that over about 4mb the video will not "auto start".
If in the "video_auto_start" action you trace the video plugin time, for a file under 4mb the value traced is somewhere around 0.3±, any thing over 4mb it traces 0 and the video will not play.
Any ideas?