Different Euphorbias have different growths habits. I've found griffithsii to be rather spreading - it aims to colonise as much ground as possible and so sends up stems at a distance, rather like wood spurge in that respect, whereas eg characias, mellifera rely on becoming quite big bushes in one space. I wonder whether it's because griffithsii dies down over winter? Mellifera gets big and is less likely to be overshadowed by anything else, whereas griffithsii has to start afresh each year.