One of the great questions of Argentina. I've not found it to be true in other parts of the world when you buy in bulk since it costs less to make 1 big package than 2 small packages, etc. That's why stores like (in the US) Costco/BJs/Sams Warehouse are so popular. Normally, price per kilo should be cheaper in bulk. Yet more often that not, it's actually *more* expensive. I believe someone posted here once that the theory behind it was that if you could afford to buy large quantities at once, you obviously had money so you should pay more. Completely illogical but this is Argentina so no surprises.
I often wind up buying several small packages of things like spices, butter, etc at Makro rather than 1 larger bag/package/whatever.
To answer your 2nd questions - buying 1 large container of something is easier and more environmentally friendly than buying multiple small containers. So I would always prefer to buy in bulk, divide it and freeze. But that's not always the most economical option.