Wow, 40%. That's pretty good. If you still want to use green I think that would work too but maybe a more solid material. Like maybe some green silk, although that may have some shine to it which may tend to cause some gradients in the green. Black may work as well.
I actually found the green you used to be unique in that most sellers use black or white. So if you think that helps you might be able to find a fabric that gives a more solid green and you can keep that 40% size reduction and still keep the color that works for you.
I think it has to do more with the green being felt or wool or something so there seemed to be a lot of shades of green. When the software that crunchs images is shrinking them it looks at pixels next to eachother. Whenever it finds a few in a row that are exactly the same or close enough that the human eye can't tell the difference it combines them all into 1. Thus maybe 5-10 pixels get converted to 1 giving a 50-90% space crunch. That's the simple explanation anyways. There are many different algorithms for image shrinking and I'm no expert by far. I just know that when there are a lof similar colors near eachother the image can be shrunk down a lot.