Is there any minimum size/weight for check-in luggage? I suppose they don't expect that people check in their wallet. But, since some things are allowed as check-in but not as carry-on, it makes sense to have a small bag for items like knifes, shampoo bottle, spare batteries. I imagine the limitation might be for technical reason, that is, the conveyor bell might not deal with items too small, too light.
If you look into an airline's terms and conditions, you will in most cases find a vague and unspecific disclaimer escentially stating that the airline can deny at their own discretion to transport certain items.
Just as an example, Lufthansa's T&Cs contain the following disclaimer, but you will likely find similar in other airline's T&Cs:
8.3.1. You must not include in your Baggage: ... 220.127.116.11. items which are reasonably considered by us to be unsuitable for carriage ... by reason of their weight, size, shape or character
Luggage handling facilities at airports are not built to handle arbitrary small items, so yes, you must expect the airline to deny accepting them as checked luggage. Contrary to maximum dimensions, which are usually specified, it is for small items up to the handling agent to decide if the item is acceptable or not.