jframe | java web application architecture based on spring mvc | Model View Controller library

You have to understand the meaning of e.getSource(), it's return all properties of button. Your condition e.getSource() == this return false that is a problem. You have to set image icon like new javax.swing.ImageIcon(getClass().getResource("path/img_name")).

Here down is generic code implementation so the logic will work on any of the nine buttons:

You need to memorize and manage the selection of a piece. You can do it by:

As explained in my comment you were adding an action listener twice so you get 2 responses for every action.
I also removed all static modifiers in the following mre:

It's really important to read the documentation, especially for something that is (to my simple brain), complicated.

If you have a read of the documentation for AffineTransform#scale

Concatenates this transform with a scaling transformation

(emphis added by me)

This is important, as it seems to apply that each time you call it, it will apply ANOTHER scaling operation.

Based on your avaliable code, this means that when the Piece is created, a scale is applied and the each time it's painted, a new scale is applied, until you're basically scaled out of existence.

Sooo. I took out your init (applied the scale within the constructor directly instead) and update methods and was able to get a basic result

Scaling from 1.0, 0.75, 0.5, 0.25and0.1` (it's there but I had to reduce the size of the output)

Edit: The behavior of the min size, growing after a resize, and becoming larger than the max size seems like a bug.
Setting the min size explicitly is a workaround it:

You cannot add Frames to another component. Frame is a top level component with a native peer. You should subclass from something else (JPanel?) instead

Also, btw what you are doing is not good design. Generally, in MVC Swing design, all view and control aspects should be in one class. Don't split the view into multiple classes unless each of those classes stands up as its own reusable widget

Not sure if I can accept comments as answers, so I posted the answer here. It was indeed changing the rectangle height to int h = ((i+1)*MAX_BAR_HEIGHT/numBars). Thanks both to @AndrewThompson and @c0der not only for the answer but the additional information as well.

Aside: @c0der gave great advice re "when in doubt, print out". I recommend using this change:

Your Unit JComponent is likely being added to mainPanel in the ActionListener and thus the GUI, but it has no preferred size and so per the FlowLayout used by JPanels, it will size to [0, 0]. FlowLayouts (and most layout managers) do not respect a component's size but rather its preferredSize. Also, revalidate() and repaint() need to be called on the container (mainPanel) after Unit has been added so that the layout managers can do their laying out of components and to allow the OS to clear dirty pixels.

To solve this, give it a preferred size, preferably by overriding public Dimension getPreferredSize() but by calling setPreferredSize(...) if you must, and by calling revalidate() and repaint() after adding the component to the container.

Better still, add the component to the container using a CardLayout tutorial, but hide it by also adding an empty JLabel, again using a CardLayout, and then display the hidden component by calling CardLayout.show(...) from within ActionListener.

Side note: don't forget the super method within your painting method:

By adding a key listener it's possible to reverse engineer the Key Stroke string: