The weight of fiber first intruded on my consciousness when I was in school to learn weaving. We would weigh the cones of yarn from the yarn closet on an old-fashioned baby scale before we wound our warps and shuttles. Then after weaving was done, we would weigh the cones again, giving us the amount of yarn to be charged on our school supply accounts. I never thought much about it other than that it had a nice housekeeping orderliness to the process.
The next time I thought about weight was when I was lugging armfuls of tie-dyed clothing from the car to my booth on the vendor's meadow at music festivals - rayon is heavy! Especially compared to the linen and silk items I was sewing because I couldn't find white linen or silk "blanks" anywhere. I promptly started sewing my own clothing items to save my back.
The third time I had to deal with the weight of fiber -- and I paid attention this time -- was at one of my first felting classes. The students were making hats, and I had put out the big coils of roving in all the glorious colors I had brought for them to pull from. They went wild, and couldn't keep their hands off this color and that (you know the feeling!) and by the end of the class, most people's hats weighed up to a half pound. Heavy, man! After that, I tested to see how little wool was needed for a felt hat and packaged 2 ounces each in all the colors for them to choose from. It was more work for me, but much more successful for the student's first experience in making hats - they were still colorful, but plenty light and warm. Eureka!
The importance of weight in handmade items has been on my mind lately as we package yarns and fiber for the shop, and make suggestions on what amount to buy to make what items. Fleece for felting is easy - I know exactly how much you need for each type of item, from years of weighing before and after, that good habit being ingrained since I went to Haywood. (Thank you, Catherine!)
But the amount of yarn, as well as the amount of fleece to spin yarn is sometimes difficult - so much depends on the designated size of the yarn, the stitch, the needle size, your style of knitting, the patterns you're using, etc, etc. So yardage is a moving target. That's one of the reasons knitters have such a tough time getting a pattern right - especially when they don't take the time to do gauge swatches. But weight, now there's a solid concept in knitting, crochet, and weaving that we can talk about.
Just like the felting student didn't realize that more wool than needed would mean carrying a half pound on her head (I'm getting a headache just thinking about it), you don't want a pound of yarn draped around your shoulders for a shawl, or two pounds of sweater hanging (and stretching) to your knees. And just like the linen and silk tie-dyed items I made were light as a cloud to pack and carry, linen, silk and wool yarns will be far, far lighter than rayon, cotton, and alpaca yarns, so you can get more yardage for the same weight.
I know that most knitting patterns will have the yarn brand, style and number of skeins to buy to make the pattern, and sometimes they will give the weights as well. Sure, you can tell something about the weight of the item from the photo - if it's lace, it's light, right? - but if you do the math on the weight of the yarn called for, you come to find that lacey scarf, made from cotton and rayon yarn, is actually 8 ounces in weight, a half pound hanging from your neck. If you made the same scarf from a wool or silk yarn instead, it might be half that weight.
So my suggestion is, find your favorite items in your closets and drawers to wear. Find a scale that does ounces and pounds, and find out how much it weighs. Then use that weight to buy your yarn. A scarf should be light in the summer and only a little heavier for the winter. A sweater should not weigh more than a pound, and so on.
After you've found an ideal weight for your knitted item, buy that amount of yarn in the size designation that you like to work in. Knit a swatch with the needles that seem to fit that yarn and the stitch pattern that you'd like to use. (start with the needles that a doubled piece of yarn fits through in a needle gauge) Knit at least a 4" x 4" swatch and extrapolate the size of the swatch in weight and size to the larger garment, and change the size of needles and maybe the stitch pattern til you get the right combination that will give you the size and weight of the finished item.
Yes, you can always buy more yarn if you need it, but the point is not to add weight, but to knit to the weight you prefer.
I know that isn't as thought-free as just following a pattern. But you will know with much more certainty that you'll like the finished piece.