When adding an image to an excel sheet it is always displayed with the default resolution. Width and height properties are set to the number of pixels respectively. That means it is not displayed with its print size but with its pixel size.
It would be better if the DPI setting in the image file would be used to adapt the display size of the image accordingly. The following formulas should solve the problem:
wb.active._images.width = xpixel * defaultXDPI / imageXDPI wb.active._images.height = ypixel * defaultYDPI / imageYDPI