Having noticed the same phenomenon in India (although it is now slowly changing and true only for small shops), i would say that the 3 hypothesis you stated seem perfectly acceptable. To what i would add that the 3rd isn't necessarily the weakest - people by products on a daily basis because of Freshness (veggies but also milk and yogurts sold in plastic packets - which get "spoiled" even when kept in the fridge), habit, and socialization. Think of the local store as a gossip hot spot and one of the main outing/acivity in the day of many housewives.
I think that it would also make sense to relate your hypothesis #1 and 2. Since purchases are little, so is the amount spent at a time (by the way here is another reason for your 3rd hypothesis - i.e: being able to afford immediate needs > the "shampoo sachet culture"), thus, the price of a bigger plastic bag in relation to the purchase does not make too much sense to the owner for whom a rupee (the price of a thicker/larger plastic bag here if i remember it right) matters as it is equivalent to the kind of margin he would make on one product.

Sorry for the length of the comment / Thanking you for the daily inspiration perfect.

While living in Silicon Valley I noticed the same in local grocery stores. After the usual "paper or plastic" question the attendant/cashier/helper started packing the goods, and the unusually small plastic bags were not saved. Usual shopping ended with maybe 7-8 bags. For heavier items with sharp corners (1/2 gallon juice), two or even three bags were put inside each other. (Which seemed almost unacceptable waste with my North-European background).

My explanation after 2 years there:
(1) smaller packages require less physical effort (loading desk -> cart -> car).
(2) with smaller packages (units) the carrier can adjust the best carrying weight. Not to walk too many times and not to carry too much.
Same applies to first use (shopping) and second use (removing waste).

Thanks for your website.

I live in Rio, and I often leave the markets with those bags. I have even spoken to some people about it, and some points that I found were that customers visits the markets (more than one) daily searching for discount on products, and those trips are unplanned (coming back from work, bank, etc).

Those bags are also the last products from plastic recycling plants, so it's better have them wasted than harder plastic, which would take more time to degrade.

the same happens in Argentina, they have the same type of bags you mention. People who buy lots of things often choose to have their groceries home-delivered.

In "Dutch" supermarkets in the Netherlands you are suppose to pay for bags, these bags are meant to be reused and are bigish and pretty strong. On the contrary if you buy in a Turkish or Morroccan shop in Amsterdam you are given (smaller) bags for free.

In Spain, where I live now, you get free bags everywhere, these bags are thin and small like the Brazilian bags you mention. Some people do use them for their home rubbish too. Spaniards often use shop trolleys, there are several models, even trendy ones.