How to set zoomFactor in selenium + phantomjs?

Question

Phantomjs Version: 1.9.7
Selenium Version: 2.40.0

Setting zoomFactor works well when I directly call phamtomjs with page.zoomFactor set in .js file:

Works well

var page = require('webpage').create();
    page.open('http://google.com', function () {
    page.zoomFactor = 0.25;
    page.render('test.png')
    phantom.exit();
});

However it doesn't work when this is set in desired_capabilities in selenium:

Does not work

DesiredCapabilities.PHANTOMJS['phantomjs.page.zoomFactor'] = 0.25
driver = selenium.webdriver.PhantomJS()
driver.get('http://google.com')
driver.save_screenshot('test.png')
driver.quit()

I also tried this, but it doesn't work either:

driver = selenium.webdriver.PhantomJS(
    desired_capabilities={'phantomjs.page.zoomFactor': 0.25}
)

What am I doing wrong?