The purpose of this change in the current beta is to make the icons visible on high resolution screens, as per the original post in this thread. I don't believe it would make sense to leave some icons small, and not visible, and other icons large. The interface, ie. icon size and visibility, should be consistent.
I understand your point of view, but for example I like to have Toolbar icons in large size, but Treebar icons in small size. I think you could make this option available to anyone who wants to use the large or small treebar icons.
