I had this from a co-BM at a wedding earlier this year. We were wearing cheap-ish (in the scheme of weddings) dresses made in China, bride asked us to give her sizes and she'd buy at least one up, if we were between sizes we should go 2 up.
The whole idea was to guarantee we had more fabric than we needed and get them altered in the UK.
I'm a size 14 so asked for a 16, other BM is at least the same size as me, but asked the bride for a size 10! Bride ordered a 14 anyway, only for it to arrive and be too small - miles to small.
Other BM went a bit mad at bride, bride took dress to get altered, there wasn't enough fabric. Bride bought new dress (size 16), still a bit too small with the seams let out but not enough time to re-do the back.
On the morning of the wedding, other BM was the last to get ready as she was being bridesmaidzilla from hell. When it came to getting into her dress, it took 3 of the BMs to squeeze her in, I'm not sure if she put on weight but she could have tried some support undies or something.
She then asked me if we could swap dresses because I must be wearing hers (even though my dress had my name pinned to it, and I'm at least 4 inches taller and my dress was much longer, and you could see where my dress had been taken in too).
She went crazy, insulting the bride, having a go at me, saying the mother of the bride was out to get her(?!) and was just the worst person possible to be around. When the bride walked in she just shut up and didn't say another thing about it - it was so bizarre!