When uploading a small image (ex. 100x150px) I noticed a few issues:
1. The featured image is literally processed/stretched to be 512x768px. This is does not look good and uses up more resources than necessary.
2. The thumbnail is stretched to 256x384px when processing the image which is scaling it up a bit.
3.. The square image (128x128px) does a little bit of stretching but not much. Not a big deal, but figured I'd mention it.
On the subject of small images, small images do not need to produce all 6 images. I do suppose however this would require a fall back method of some sort. I've seen other site's produce less images when they are smaller. I figured I'd mention this to have another method to save space where extra files don't really do anything.
Attached to this post is a small image to experiment/see this with.