Screenshot chrome.tabs.captureVisibleTab() reducing filesize

I'm making a browser extension that takes a screenshot of certain pages and saves them as JPEG.

Here is my code to take the screenshot:

var screenshot = {
    content : document.createElement("canvas"),
    data : '',

    init : function() {
        this.initEvents();
    },

saveScreenshot : function() {
    var image = new Image();
    image.onload = function() {
        var canvas = screenshot.content;
        canvas.width = image.width;
        canvas.height = image.height;
        var context = canvas.getContext("2d");
        context.drawImage(image, 0, 0);

        // save the image
        var link = document.createElement('a');
        link.download = shotname + ".jpg";
        link.href = screenshot.content.toDataURL();
        link.click();
        screenshot.data = '';
    };
    image.src = screenshot.data; 
},

initEvents : function() {
    // send an alert message to webpage

    chrome.runtime.onMessage.addListener(function(message,sender,sendResponse){
        console.log("Got a message!", message);

        if(message.action === "screenshot"){

            shotname = message.name;

            chrome.tabs.captureVisibleTab(null, {
                format : "jpeg",
                quality : 10
            }, function(data) {
                screenshot.data = data;
                screenshot.saveScreenshot();
            });
        }
    });
}
};

This works, although my issue is that regardless of what quality I set the JPEG to save at, it results in a ~400kb image. (the image will always be of dimensions 1280px x 720px). Changing quality from 100 to 60 to 40 to 10 made very little difference in filesize.

Compare this to "Save for Web" in Photoshop on the same size image, I get a 30kb JPEG and it has no noticeable lack in quality.

Is there something I'm missing here, besides the fact that Photoshop likely has a better compression algorithm?

How can I get nice quality images at a small filesize using Chrome's captureVisibleTab?

If it's not possible are there any workarounds I can use to automate this within a Chrome extension?