Hi Jens,
Metallurgy is probably one of the most difficult aspects of studying these weapons for me, as it is obviously so technical, but your observation is well placed and seems like a logical question. In rereading some of the fantastic work of Ann Feuerbach she notes certain differences in the shapes of the crucibles, i.e.conical (eggplant) shaped in southern India, and elongated, pear shaped or light bulb shaped in Sri Lanka, while Hyderabad crucibles were cylindrical, similar but shorter than those in Central Asia.
As a layman, it is my impression that the shapes of the crucibles must have had something to do with the manner of stacking them in the furnace, as well as has been mentioned, some consideration for heat transfer . It seems that since the cakes of metal needed to be reheated to be forged the molten or redhot metal could be drawn into the necessary elonged shape fairly easily.
This sounds like a question that Ann or some of the metallurgists here might have an answer for. Now you have me curious too!!
All the best,
Jim