Just that really. I've seen it on a few threads recently and it's been on my mind how insulting it is, and how often it's said! I'm getting married this year and my bridesmaids have flattering dresses. They are also slimmer than me and don't look haggard from having a small child, along with most of the women under 35 who will be at my wedding. So objectively, whilst I will definitely be the most bridal looking person there, and one of the two happiest, I won't be the most beautiful.
But that's fine, because my future husband will think I look beautiful, and as for the guests, there's no danger of like "well I thought that woman in white with the bouquet was the bride, particularly as I've known her 15 years but then my eye was caught by this beautiful 23 year old with a stunning figure and I decided that perhaps she was the bride and I should take more photos of her and ask her to cut the cake because she's so much shinier."
Could you imagine if men were told not to outshione each other? It's just more pitting women against each other. Surely nobody can "outshine the bride" unless you believe that somebody looking more stereotypically attractive makes somebody more worthy of attention?