Performance and file-size with embedded images

Hello,

I have an ipe document that contains about 15 images in .png format, 
rendered at 100 dpi. This document takes 16 MB as .pdf and 21 MB as 
.ipe, and can be edited smoothly.

However, when I replace the images with higher resolutions (300 dpi), 
then the file size changes to 183 MB for .ipe, and 135 MB for .pdf. 
Furthermore, even just selecting an image with a single click takes 
several seconds. The document becomes essentially un-editable.

Why is this? Shouldn't the file size grow at most by a factor of 3, if 
the image resolution is increased by this factor? And why this extreme 
slowdown?


Thanks,
-Nikolaus