No, they would be essentially indistinguishable as far as online video watching went, and just in general as well.

Now, the 15" has a Core i5 or i7 depending on the configuration, and that could very well make a difference in some areas. But if you're mainly looking for things like web browsing and youtube video watching, aside from the extra 2" of diagonal screen space on the 15", the three are going to be indistinguishable from one another.