How are clothing sizes for babies so different?

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I have just bought the next size up for DS who is 9 months (9-12 month) Sainsbury's vests fit him with room to grow - great. Mum has purchased vests from Asda in 12-18 and they are smaller than the Sainsbury's 9-12. She washed them to be helpful but just wondering if I could take them back. They will no way fit a 12-18 mo baby unless they are tiny. DS is a chunk but he isn't huge on the length scale. Has anyone else noticed this?

Yeah it's a pain. Generally the more expensive they are the bigger the sizing. Asda has always been about right for DS as he is long and skinny he grows out of things by length not width.

He is 3 1/2 now and over summer has been wearing some shorts in size 18-24 months as he finally fits into them!!!

So annoying! Tesco is tiny! I compared a 0-3 month baby grow to a mothercare one (back in the day) and the mothercare one looked a good few sizes bigger! Definitely the more expensive the bigger the size.