The Mystery of the Half-Plate Biryani

There are these string of roadside shops in Indiranagar that sell chicken biryani from huge dekchis. The biryani-s are of course of typically Chennai quality, implying they are a strange mix of Hyderabadi biryani mingled with local flavours (which means that some Chennaite, long ago, had gone to Hyderabad and tasted Biryani there, and having liked it somewhat decided that it might be actually made palatable by adding some local spices (hint of tamarind, anyone?). Anyhow thus started the Chennai biryani and it prospered and flourished in this land of idli-s. And just try telling someone that this is not really what Biryani is supposed to taste like and you will met with a long cold stare). Just in case this sounds exotic, a word of caution, try it only if your souls yearns for Biryani and you are ready to forget how real Calcutta biryani (maybe I should say Lucknowi Biryani, but forgive me my sins, Calcutta Biryani is the name that I call it by) tastes like in leau of the satisfiction of being able to say that "I had biryani tonight"!! (Which moments, if you have lived in Chennai for four years, as I have, does come uncotrollably once in a while.) However, it is not the quality or taste that is the mystery (although they could warrant a post by themselves), but rather the menu-card. The menu of all these shops reads:
1/2 Plate Biryani : Rs. 30
1/4 Plate Biryani : Rs. 18
The question is what happened to the full plate of Biryani? Did the stallkeepers get together in some meeting and decide that 1/2 a plate of this stuff is the maximum limit that can be allowed for safe consumption? But in this case they could simply have defined that as one plate and be done with it. Not to mention the fact that they would be saving the painting charges of the "/2" part (perhaps I should clarify that the menu is just painted in big lettering on the wall) and thus could feel pleased with themselves. But maybe there is some strict rule governing the quanta of 1 plate of biryani decreed in some holy edict, and since "Guru-r nished aache" is a strong enough reason for any action, it was sufficient for them to adhere to the straight and narrow path and forego the lure of the money they would have saved. After all what is the value of a "/2" in the face of eternal damnation?

P.S.:
[1] The proprietors of Shiraz/Royal and the like could do a roaring business if they opened shop here. And I don't even want any share of the profits for pointing out this golden business opportunity, just the chance to have real Biryani shall suffice.
[2] The Indiranagar biryanis actually have a point in their favour. They do not add curry leaves in their Biryani, which is enough to make one grateful for small mercies.
[3] As you can perhaps make out, I live in Chennai and am always on the lookout for decent Biryani joints, So if you know one and can point it out to me, you shall earn my everlasting gratitude. And I could also point you to the only shop that I know of that serves decent Calcutta Biryani (it's too far from where I stay for me to go regularly, alas), so its a win-win offer.