Its largely conjecture that Ip Man had studied with Leung Bik at all.

And if Leung Bik was smaller than Chan Wah Shun, why would he use high kicks and throwing?

I would speculate (guess) that if Ip Man did really meet Leung Bik, Leung would have taught him to use his body more efficiently to control the center of gravity, and then be able to throw and/or kick or strike at will. And also, his Si Hing would not brand Ip Man a traitor because he studied with his Si Bak (elder martial arts uncle), and it is in keeping within the family. All of it makes no sense in the movie.

Lastly, if someone studies outside of the family and uses the skills to show the family they are lacking in certain skills, it would make sense to the family to grow and develop counters or adopt the skill which defeated them. Chinese have been open minded about this for centuries.