Often hard to tell volume just by looking and CCs do not tell the whole story.
I don't think you can begin to guess which head is which volume here.
One is 23-25cc and another is 18-22cc, I cannot remember exact numbers now, but hard to tell just by looking.
So the stock Blaster head on the right is 23-25cc. Squish distance was over 3mm. I filed 1mm off the head (still over 23cc IIRC) and ended up with detonation. (1mm = 0.0394" ~0.040")
The head on the left is a chamber shape I had cut in a spare head. With this shape I was able to reduce the volume as low as 18cc with no detonation.
This is how you CC an head:
Note that the frosty amber liguid is for the technician and is totally optional.
A greased plate of any flat liquid tight material (glass or aluminum etc) and water is measured into the sparkplug hole. When it is filled to the bottom of the threads, you have your volume. You can calculate the headgasket and piston deck height volumes.
Different chamber shapes return different results. One of these is a low RPM torquer and the other is a peaky revver.
I am kind of excited to try this same sort of experimentation on the Grubee.
Just throwing an extra headgasket on my seem to be a quick and easy answer, but often doesn't work because of issues with quench distance.
Steve