Mammillaria luethyi question

[Fatich]

I ve got 3 grafted Mammillaria luethyi, one of them is really compact and the other 2 are not compact.
What causes it?

[DaveW]

Plants do tend to grow out of character on grafts, or even on their own roots in cultivation due to richer soils or fertilisation, since in habitat harsher conditions and slower growth keeps them more compact. You have to remember you are putting the rootstock of a far larger and more vigorous plant on what is normally a small growing scion.

There is a good article on Mamm luethyi by Paul Hoxey in Cactus Explorer 3, pages 30-36. showing it's size in habitat. Link below:-

http://www.cactusexplorers.org.uk/journal6.htm