You are not the only person ever to ask the question.... many years ago I did an experiment I tied one pattern with varied thickness's all the same price at the same show. Guess what resulted not what I thought the full bodies sold more... I guess that’s why everything is SUPERSIZED ... getting the biggest for the same price. If you look at my fly bag ... I have patterns like sand eels very sparse and I have mummy chubs very fat and full of material. It's all on what works for you and the FISH... just a hint... if you can read the newspaper thru the flies’ body you are sparse. I don't have any specific to guide you on the density of the fly... but be assured the fish will let you know right away. The way to do a crude test is to fish with three of the same patterns, fat, sparse or in between... yes all on the same leader you will find out quickly which size and shape or color the fish DNA are keying on. Sometimes it really doesn't matter if the fish are in the feeding mode all will work... even a piece of flash on hook will catch. It's when they are fussy or just not in a positive feeding mode... you need to find the right flies to turn them on to feed.
I guess to sum it up ... go with what you know ... try it until it works for you. I believe there is no true answer , sorry but that's the fun of fishing.