You need to specify the default value for Shape.toArrow (or Shape.fromArrow) if you want that shape to be considered an arrowhead. Values such as “Standard” or “OpenTriangle” are commonplace. See GoJS Arrowheads.
Due to your data Binding, if you do not provide a value on the link data object, data.toArrow, or if the value is not a proper arrowhead name, then the Shape.toArrow property won’t get set, so it won’t be an arrowhead, and it will appear with its default values. That includes getting a 100x100 size, which we do intentionally so that developers can see that they didn’t initialize something about the Shape.