@elanchezhiyan,
With a few lines of JavaScript code, you can retain the size of the exported image with the help of devicePixelRatio. Take a look at the code-snippet given below.
chart.set("width", 1200 * (1 / window.devicePixelRatio), false);
chart.set("height", 1800 * (1 / window.devicePixelRatio));
chart.exportChart({ format: "jpg" });
Please refer to this Stackoverflow thread for more information on handling device / browser scaling in JavaScript.
—
Vishwas R
Team CanvasJS