Run the code below. The JSlider thumb is twice smaller:
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import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.plaf.metal.MetalLookAndFeel;

public class JSliderTest {

    public static void main(String[] args) {

        System.setProperty("sun.java2d.uiScale", "2");

        SwingUtilities.invokeLater(() -> {

            try {
                UIManager.setLookAndFeel(new MetalLookAndFeel());
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
            JFrame frame = new JFrame();
            frame.setSize(300, 300);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            JPanel panel = new JPanel(new FlowLayout());
            JSlider slider = new JSlider();

            panel.add(slider);

            frame.getContentPane().add(panel);
            frame.setVisible(true);
        });
    }
}
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