If you ever dissect an uncompressed pages file there's a lot of junk in there you really don't need. My guess would be that this is a case of that, Apple including the default fill pattern when it need not be included.

Uncompressed, an empty pages file includes about a megabyte of XML for a 3x3 sheet with tons of absolutely unnecessary default metadata about the sheet as well as each individual cell. Pages really isn't very good at knowing what it actually needs to include.

I spent a few days dissecting the format a few years back at a previous job for a data processing tool. We decided it wasn't worth our time.

As for the size, some things simply don't compress well in PNG.

I guess the other color image fills are just variation of the original blue image.