actually, i don't use canned soup or spaghetti sauce. however, i will sometimes use purchased cooking stock.
the issue is that i don't have recipes. and i tend to add more ingredients to accomplish the end result. and doing so, the recipe grows in quantity. also, i think i just never seem to be very good at estimating quantities of food. plus, in the back of my head, i tend to think that i might be sharing, even though i really do live alone.
oh, and i suppose the blonde answer you might want to hear is that i'm not smart enough to figure out how to scale my recipes? : ) the free spirited gypsy in me doesn't really care that much, because no pot of soup that i make ever tastes EXACTLY the same. it's always going to be a tiny bit different, on the basis of seasonal ingredients, what extra ingredients i have on hand, and what i am especially having a craving for.
for example, i often have a 1/3 - 1/2 bottle of wine in the fridge that i will use for a big pot of soup or pasta sauce, so i will use that in substitution for part of the water. when people ask me for recipes, i always struggle to provide one, since i seldom have one for reference. just ingredients and ratios.