We currently set the default CompressedClassSpaceSize to 128M - much lower than the current upstream default of 1G, and still much lower than the possible encoding range with 4-byte-headers of 512M. I think I did this earlier to make some tests pass and to address a problem with CDS which also wants a smallish share of it, but I have in-fact only hidden a bug.

The way it should work is to set the CompressedClassSpaceSize to 512M and let CDS take its share and use the rest for class-space. This is infact implemented by JDK-8332514 since a while. However, there is a bug there that prevents it from working with 4-byte-headers: it has 4G encoding range hard-coded. This needs to be fixed.