When you use Chart.Export method to export a chart as an image to manage image size you have to work with 2 parameters:
1) resize chart image before calling export method, then call export, then restore original image size (i.e. it looks great in Excel but resolution too small when exported as image file, so you need to increase chart size, export it and restore the size)
2) check ActiveWindow.Zoom parameter, as it’s also affects image size.
Be sure you always have the same combination of both parameter values before exporting chart and you will get image with the same resolution.