Shri TVG,
AFAIK, the ancient customs as seen from our scriptures, talks about a ceremony or samskaaram known as samaavartanam for the male members of the three higher classes (castes) viz., brAhmaNas, kShatriyas and vaiSyas. This was to be performed when a young man, after his "gurukulavAsam" and learning of one veda or more vedas, and having been certified by the guru as his having completed his study of veda/s successfully, returns to his native village.
The local people on hearing about the returning BrahmachAri, formally receive him at the village border, offer him a seat there (possibly below a banyan tree) and then he takes a ceremonial bath. After that he returns to his seat and then he is invested with the skin (hide) of a black buck (Krishna mrugam) - this skin is known in sanskrit as "KrishNAjinam" - by an elder and this KrishNAjinam is placed on the Brahmachari's left shoulder and joined and held under the right side, exactly like how the pooNal is worn. The "maan thOl" today (which is mostly a pice of goat or sheep skin, because black-buck killing is a criminal offence today!) is in lieu of the original KrishNAjinam.
I have not read anywhere that when the above practice was in vogue, the practice of wearing a poonal made of cotton thread was also there. But for any vaideeka karma, including the performance of various "yAgams", the yajamaanan and his wife, both, had to wear a "vasthram" - a cloth at the waist - and also an "angavastram", a cloth to cover the upper part of the body. This angavastram if worn in a flowing manner, as we normally do now, used to be very inconvenient and also susceptible to fire accidents probably. So, it came to be worn as a belt-like "paTTi". The Nambudiris of Kerala, who are also brAhmanas, wear this angavastram after making it into a twisted coir-like thing. The attached image shows this.
I am of the view that it was this "angavastram" which became today's pooNal.
This post by our esteemed member Shri kgopalan explains how the poonal is supposed to be made. (I remember my maternal grandfather, a vaadhyaar, had two or three cotton plants in his compound and used the cotton from those plants only, for making poonals in the house and offering them to others.) But today, the most preferred thread by vaadhyaars is the tyre-making cotton cords which will be very strong and therefore, more lasting; many vadhyars also get ready-made poonals from Madurai which saves them lot of trouble, but these are made by people of any caste under any conditions of "Shuddham" and thus, these are 'secular' poonals.
Regarding thickness of vaishnava poonals I don't know the reason. Hope this reply is credible enough.
