With JxBrowser you can zoom the web page content using Browser.setZoomLevel(double zoomLevel), Browser.zoomIn(), Browser.zoomOut(), Browser.zoomReset() methods. Please note that zoom level can be applied only for the completely loaded web page. You need to wait until the web page is loaded completely and only then you can zoom it: 

browser.addLoadListener(new LoadAdapter() {
    @Override
    public void onFinishLoadingFrame(FinishLoadingEvent event) {
        if (event.isMainFrame()) {
            event.getBrowser().setZoomLevel(2.0);
        }
    }
});

The zoom level is configured for each domain separately, so if you set zoom level for the a.com web page, it will not be applied for the b.com web page.

import com.teamdev.jxbrowser.chromium.Browser;
import com.teamdev.jxbrowser.chromium.BrowserContext;
import com.teamdev.jxbrowser.chromium.ZoomService;
import com.teamdev.jxbrowser.chromium.events.FinishLoadingEvent;
import com.teamdev.jxbrowser.chromium.events.LoadAdapter;
import com.teamdev.jxbrowser.chromium.events.ZoomEvent;
import com.teamdev.jxbrowser.chromium.events.ZoomListener;
import com.teamdev.jxbrowser.chromium.swing.BrowserView;

import javax.swing.*;
import java.awt.*;

/**
 * The sample demonstrates how to modify zoom level for a
 * currently loaded web page. Zoom level will be applied to
 * the currently loaded web page only. If you navigate to
 * a different domain, its zoom level will be 100% until you
 * modify it.
 */
public class ZoomSample {
    public static void main(String[] args) {
        Browser browser = new Browser();
        BrowserView browserView = new BrowserView(browser);

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.add(browserView, BorderLayout.CENTER);
        frame.setSize(700, 500);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);

        // Listen to zoom changed events
        BrowserContext context = browser.getContext();
        ZoomService zoomService = context.getZoomService();
        zoomService.addZoomListener(new ZoomListener() {
            public void onZoomChanged(ZoomEvent event) {
                System.out.println("event.getURL() = " + event.getURL());
                System.out.println("event.getZoomLevel() = " + event.getZoomLevel());
            }
        });

        // Modify zoom level every time when main frame is loaded.
        browser.addLoadListener(new LoadAdapter() {
            @Override
            public void onFinishLoadingFrame(FinishLoadingEvent event) {
                if (event.isMainFrame()) {
                    event.getBrowser().setZoomLevel(2.0);
                }
            }
        });
        browser.loadURL("http://www.teamdev.com");
    }
}