Seed stitch is what I and other British knitters call moss stitch, but in this post I'll be quoting an American writer, so for the duration of the post, I'll refer to it as seed stitch. (I'm not going to be consistent, though - I cannot bring myself to call stocking stitch 'stockinette', so I won't.)
Back in 2010, not long after I started knitting again, I came across a quote from The Principles of Knitting by June Hemmons Hiatt: "Seed stitch is 30 percent shorter than Stockinette, and 18 percent wider." As well as introducing me to that wonderful book, the quote was an amazing revelation. I knew that rib is used for welts because it's stretchier than stocking stitch, but apart from that, it had never occurred to me that different stitch patterns behave differently - I thought that the choice of stitch pattern was all about what the resulting fabric looks like. I knew that seed stitch is slow to knit, because of the constant switching from knit to purl, but I hadn't really understood that it's also because it takes more rows to the inch than stocking stitch. (Yes, I know I should have.)
Since I came across her quote about seed stitch, I have read June Hemmons Hiatt 's book, and bought my own copy of the revised second edition. And I have generally been much more aware of the characteristics of different stitch patterns.
Just recently, I have been thinking about various stitch patterns, and decided to do some experiments of my own. I knitted swatches of different stitches, all using the same yarn, same number of stitches, same needles, same length of yarn. Here are two of the swatches, in seed stitch and stocking stitch.
It's easy to understand why seed stitch takes more rows per cm. than stocking stitch - the vertical lines of stitches (wales) are kind of concertinaed, producing a thicker fabric. I can't really see why it should be wider (but then as it isn't, for me, I'm not well placed to understand it). So I don't know why there is such a discrepancy between my swatches and the account in Principles of Knitting. Perhaps different knitting methods affect the width (i.e. yarn in right hand v. yarn in left hand, picking v. throwing, or whatever you want to call them). Or perhaps it's just that different people knit differently. It's intriguing.
I knit with the yarn in my right hand, the throwing method (aka English style). If any reader knits seed stitch that is wider than stocking stitch, please tell me what knitting method you use - I'd love to know.