Sitecore ensures that images are scaled to their use case; and thus the sizes below specify the dimensions that images have to be in; in order to look optimal on the site. Images cannot be smaller than specified, as this will cause quality loss or misplaced images in relation to layout.
Image compression and pixel density
All images for web should have 72ppi/dpi, which translates to 72 pixels per inch. Higher pixel density will only cause images to load slower without gaining any considerable quality increase.
As an editor, it is important to compress the images before uploading them. Ideally, any image should not be heavier/larger than 100kb. This is almost always possible without visible quality loss.