I did it. Fast, like a bandaid, so it wouldn't hurt. It did anyway.
Thank you all for the advice and support. I did email Cynthia (from the comments), only to discover that her peplum, which she thought was too big, was smaller than mine. By a lot.
Still, wanting to be sure, I took Deep Purple with me to my knitting group this morning, to get an unbiased opinion from knitters not emotionally invested in the project. They laughed. Out loud. It was suggested that it might make a good table skirt.
And in case you're still not convinced, I decided mid-rip that I really ought to lay it out flat and measure the thing, since it was off the needles anyway.
My dining room table seats eight comfortably; ten in a pinch. I could always use it as a table runner, I suppose.
I did, at least, think to take complete measurements of every part of the peplum before I ripped. I measured and noted and calculated. So I guess you could say I just made the world's largest gauge swatch.
I also discovered source of the problem. As I mentioned before, the gauge for the peplum is not given in the pattern, nor is the finished measurement of the bottom of the peplum. But there is a measurement on the schematic (in the wrong place, but there all the same) for the top of the peplum, where it joins the bodice. There is also a stitch count for the top of the peplum. If I were the sort of person who thought ahead in this way, I could have figured out the gauge for the peplum by comparing those two numbers before starting to knit. As I am not that sort of person, I only discovered this connection today. Taking those two numbers, the required gauge for the peplum works out to 7.88 stitches to the inch. This is in aran weight yarn. My gauge for the peplum in aran weight yarn on size 6 needles is 6.5 stitches to the inch. Over 428 stitches, that is a difference of 11.5 inches. Even if I had made the size small to begin with, there is no way my gauge would have given me a finished product that corresponded to the actual or perceived size of my body.
So the next time you wonder if gauge really matters, the answer is a resounding YES!
But I am a little miffed by the whole experience. The pattern, to be honest, sucks. I know; I don't like to criticize other people's creative work, either, but if I had paid for this, I would be royally peeved. (For those who don't know, this was a Vogue Knitting "cover pattern" bonus from their anniversary issue, and is available as a free download from the VK website.) But this pattern is a Debbie Bliss design, which I understand is a reprint from one of her books. If I had paid for a pattern book to get this, I would be outraged. The only gauge given is for the seed stitch part of the bodice, the schematic does not show measurements for several important parts, and some of the measurements that are given are clearly incorrect. But what really confuses me is how anyone could ever get the required gauge with aran weight yarn. 20 stitches and 30 rows in seed stitch is almost doable (although I had to use a size 5 needle to get close), but nearly 8 stitches to the inch for the cable section? Most of the successful finished projects I've seen on Ravelry used worsted weight yarn or lighter, not aran. And most of those were modified to make them smaller. So I am baffled as to what the designer had in mind with this one.
But at least I got "free yarn" out of the deal.