Whether you're an extract or all-grain brewer, your wort contains a variety of different sugars. Unless you've adjuncted your wort heavily, there will be more Maltose than any other kind of sugar. But there will also be varying amounts of Maltotriose, Glucose, Sucrose and Fructose along with various non-fermentable dextrins. The combined total will determine your original and final gravity.
In rating the sweetness of the various sugars, sucrose is rated as 100. Other sugars sweeter than sucrose are ranked higher than 100 and sugars less sweet are ranked lower. Sweetness is detected by our taste buds, but there is no exact test for measurement for it. Some people detect sweetness in lower concentrations than others. Not all people will agree in their estimates of the relative sweetness of the sugars, so similar tests can yield different results, as the existing tests seem to prove.
Biester and Wood, University of Minnesota :
Sale and Skinner of the Bureau of Chemistry (Water and Beverage Laboratory) :
• Sucrose 100.0 • Maltose 32.5 • Lactose 16.0
Paul [sic] in comparing the sweetness of several sugars :
• Sucrose 100 • Maltose 50
(Data from "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe)
• Sucrose 100 • Lactose 28
From looking at the studies it seems that maltose may be about half as sweet as sucrose, and lactose might be about half as sweet as maltose. You'll note that maltodextrin doesn't show up in the comparisons. It is a common food additive and is classified as a sweet polysaccharide. While containing sweet qualities, maltodextrin is just barely sweet when compared to the others. At least one of the major artificial sweeteners relies maltodextrin as filler. It is also used as a thickening agent in sauces and salad dressings. It can contribute a little sweetness to beer, but it mostly adds body.
The interesting thing is what happens to these sugars during fermentation, relative to their sweetness: the sweeter a sugar seems to be the more (easily) fermentable it is. The yeast find it easier to consume less-complex sugar molecules that we perceive as being sweeter. This means that the sweetness from sucrose and maltose disappear during the ferment as the taste of the beer "dries out," unless there is so much sugar that it can't all be converted to alcohol by the yeast. It seems like if you're mashing hotter to increase residual sugar, you'll have to play around for a while before you get any notion of the sweetness relative to temperature. In any case, all of the sweetness from the more complex lactose and maltodextrin remain.
Why is this important? If you want to add body to your beer without adding undue sweetness, add maltodextrin. If you want to add sweetness and body, use lactose.
For sweet stouts, the recipes I've found use a pound of lactose. That seemed like a lot to me. Let's say that on a theoretical sweet stout recipe without lactose, the residual sugar is 1.012. Not bone dry but not terribly sweet. Adding 12 ounces of lactose would bump it up to 1.018, and pound would take it to 1.020. But that's just the body. If we look at the relative sweetness of lactose being about half of maltose, maybe we're looking at an effect on perceived sweetness that is something like 1.015 for a 12 ounces? And a pound might get us somewhere around 1.018? Now adding pound doesn't seem quite so bad.
After doing all the theoretical muddling, I found that 12 ounces will still plenty sweet. At least to my taste.