How to Set the control size change after the Delphi window is maximized

Set the anchors attribute of the button.
You can use this attribute to set whether the boundary of a class changes with the parent class. The default value is akleft + aktop, that is, the left boundary and the upper boundary change with the window. That is to say, if the position of the window moves, the button will keep the distance between the left and top boundary and the window.
 
For example, if you put a button in the lower-right corner of the window and set the anchors attribute to [akright + akbottom], no matter how you adjust the window, the button remains in the lower-right corner of the window. (That is, keep the distance between the right boundary and the lower boundary unchanged from the window boundary)
If [akleft + aktop + akright + akbottom] is added, the button changes with the size of the window.