I do not know if it's a mistake, but if the anchor in both direction is then the view should change in proportion, right?
In my example, but is not so.
It behaves correctly. Setting the anchor to BOTH means that:
1. The distance between the left side and the parent left side is constant.
2. The distance between the right side and the parent right side is constant.
As a rule of thumb BOTH is relevant for large views in the middle where the two distances (Left and Right values) are relatively small.
In this case you need to use a small script (two SetLeftAndRight calls).