java on mac - gui layout problem

I am working on an application that uses a GUI and works well on linux and windows, but there is a problem on mac. The layout of the GUI is not as desired. The main JFrame uses a BorderLayout, and there is a JPanel in the CENTER region, a toolbar in the NORTH region, and a JTabbedPane in the EAST region. The JTabbedPane containes a grid of JButtons (that are layout using a GridLayout). The problem is that the east-pane is too wide on mac. It means that the buttons in the grid within the east-pane are too wide. There is not such a problem on windows and linux. That's why I am not sure what to do to fix the problem.

I tried to put less buttons in one row of the grid (within the east-pane), but the pane is still of the same width and the buttons are even wider. That's why I think that the problem is related to the BorderLayout on mac (although I am not sure).

Does anyone have any idea how the problem can be solved ?

Thanks !