Mammillaria plumosa, 3,5 years old

These seed grown 3,5 years old Mammillaria plumosa are now bigger than my hand. They’re currently staying in a 15 cm pot, but it won’t be long before they need an even bigger pot. This is the fastest growing cactus I have ever grown from seeds. There are actually two plants in there. The big bulb on the second picture (top right near the white string hanging from the wall) is the brother of the big mass that turned out to be the faster growing seedling.
I wonder how big this plant is going to get.

Please share to help this blog grow!