In the books on our kitchen shelves there are many recipes that call for tomato paste. All that I have seen call for two tablespoons. As best I can tell, the standard American tomato paste can holds about four tablespoons. As a consequence, I have often left half-used cans of tomato paste in a refrigerator, secured with plastic, then found them weeks later with mold growing on them. I do understand that it would not be economical to sell two-tablespoon cans of tomato paste.
We could double the recipes, but then we would be making space in the refrigerator for another casserole, not another small can. We could simply use twice the tomato paste called for by the recipe, and sometimes we do, generally without bad effect. I suppose that the best approach would be to put the remaining contents of a can into a plastic container and freeze that. However, I don't know whether I'd remember to look for frozen tomato paste before opening a new can.