The JTextArea font is set to half the size of the JLabel font, if viewed on Windows, with Windows Look and Feel and a HiDPI display.

Steps to reproduce: 
Run the code: 
================
import javax.swing.*;
import java.awt.*;

import static javax.swing.JFrame.EXIT_ON_CLOSE;

public class JTextAreaTest {

    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }
        SwingUtilities.invokeLater(() -> {
            final JFrame frame = new JFrame();
            frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
            frame.setBounds(500, 500, 300, 200);
            frame.setLayout(new BorderLayout());
            final JLabel label = new JLabel("Regular Label");
            frame.add(label, BorderLayout.NORTH);
            final JTextArea textArea = new JTextArea("JTextArea Label");
            frame.add(textArea, BorderLayout.CENTER);
            System.out.println("Label font: " + label.getFont());
            System.out.println("TextArea font: " + textArea.getFont());
            frame.setVisible(true);
        });
    }
}

===============

Screenshots are attached.

System Output for Windows Look and Feel is:
Label font: javax.swing.plaf.FontUIResource[family=Tahoma,name=Tahoma,style=plain,size=21]
TextArea font: javax.swing.plaf.FontUIResource[family=Monospaced,name=Monospaced,style=plain,size=13]

System Output for standard Look and Feel is:
Label font: javax.swing.plaf.FontUIResource[family=Dialog,name=Dialog,style=bold,size=12]
TextArea font: javax.swing.plaf.FontUIResource[family=Dialog,name=Dialog,style=plain,size=12]

Note that the standard font sizes are too small. They don't respect the fact that we have a HiDPI display at all. However, the Windows Look and Feel font sizes for JLabel and JTextArea don't scale the same way, which is perhaps even more unfortunate.