I agree with Pino, I think that centre of heartwood stored energy, Thin cuttings mean that limited energy stored in heartwood area to overcome challenges to grow.
It is very hard to get cuttings here, I am able to get some thin dry cuttings, like Lauretta,Improved Celeste, Osborne Prolific, Black Mission, I use basic bag method, My house was water leak by previous owner with high mold counts in air, it means that it will be high failure rate with cuttings.
so it is amazing that Vijgenboom 's toothpick cuttings, it grow well, great job!