The size of the EMF file depends on the contents of the EMF file. The contents of the file depends on the command that the printing application send to the printer driver. This issue depends on the printing application, and there is nothing that could be changed in the printer driver to influence that.
The EMF file type is a different issue than raster images because it contains commands. The size of the EMF file depends on how many commands it has in it.
The size of an image, is the width*height*number of bytes for one pixel. This is the original image (Bitmap, BMP). All images with the same dimensions and color depth will have the same size, regardless of what is in the picture (a white page or a complex image)
If it is compressed, then the size depends on the content, how the actual image can be compressed. (white page usually will be better compressed then an image)