It's an unfortunate optical illusion: your eyes register lines that go up and down as narrow, and lines that go across as wide. If your clothing has horizontal stripes, the part of your body those stripes are covering is going to look wider. The smaller the lines and the closer they are together, the more powerful the illusion.
You don't have to part with your favorite stripey items just yet — just make sure the lines are proportional to your size. Larger stripes work best on larger sizes.
You can also use stripes to your advantage to accentuate certain areas that you want to enhance. If you're small-boobed, for example, wearing a top with horizontal stripes across your chest can give your bust size a little boost.

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