The way we have our docking windows is that when you drag a tool window out of a docked state, it tries to float the tool window with the same size. Then when you drag back to a dock guide to dock it, it tries to dock at the same size as the floating state, but I believe it caps that at around 50% to avoid completely taking over the space. The design behind all of this is that it figures you sized your tool window to a certain size and it does its best to keep it at that intended size instead of randomly increasing it.
To sum up, if you have a tall tool window and you dock it at the bottom of a tall docked tool window, it will take at most 50% of the height. If you have a more "landscape" tool window (200px tall and wider than that) and dock that at the bottom of a tall docked tool window, it will try and keep that same floating height in what is dropped. Thus it could end up being 20% in that case.
As for Visual Studio, it doesn't appear to always be 50%/50%, which you can see if you start docking three or four tool windows in a row. And it seems to change based on the height of the tool window (in my tests when docking vertically), yet it doesn't match the same size as the floating tool window, which seems wrong.
We are open to further discussing this if after the explanation above you still would like to see changes, or at least options for change.