For example assume I have 2 video files of similar length while the first one is 100Mb and the second one is 30Mb. Does the 30Mb one will load faster on Youtube?
NOTE: I'd like to know for sure, not assumptions.
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While I know you'd like to know this for sure, no one knows except those who implement the buffering at YouTube's backend. At our institute, we also evaluated various buffering scenarios, specifically targeted toward YouTube streaming. So far, the buffering model seems to be highly influenced by factors you can't estimate – with influences of the actual video bit rate and the download link bit rate.
In general though, the YouTube buffer first of all works based on time, not on data. It seems there is a buffer of around two seconds that will be filled first, then the video will be played.
In your specific case, a 10Mb (Did you mean Megabit?) video will not be buffered faster than a 30Mb video. It rather depends on the speed of your internet connection.
If you'd like to know more about YouTube QoS and QoE, I redirect you to the publications of:
See this technical paper, especially section 2.3.1 (Inﬂuence of Video Bit Rates) and the conclusion: