Dress sizes have expanded dramatically over the decades, according to experts.
A size 10 today would have been classed as a size 14 in the 1970s, meaning larger women now fit into smaller sizes. Health campaigners have criticised the change, saying shops are misleading women about their weight and are "normalising" obesity.
The British Standards Institution revealed that "size inflation" has taken size 10 clothing from an 81cm bust and 86cm hips in 1974 to 86cm and 94cm now - an increase of two dress sizes.
It means that while on the label Marilyn Monroe and American TV star Kim Kardashian would wear the same size, they are in fact very different shapes. Miss Monroe was a size 14 in the 1950s, but would be a size 10 if she were alive today.
And Miss Kardashian, who is a size 14 now, would have needed size 18 clothes 40 years ago.
Charlotte Stirling-Reed, a nutritionist, said: "I do think this is a cause for concern and it contributes to what we refer to as the 'normalisation' of obesity.
"The public may be less encouraged to change if they don't think there is a need for them to do so."
Clare Rayner, of consultants Retail Acumen, said: "People are getting larger, so clothes will have to grow with us." The average dress size for a British woman is now a 16 - a size 20 in 1974.
The criteria for dress sizes are decided by a committee every five years. The BSI says they are only a guide and shops can adapt sizes to their customer base, so "there can be no true 'standard' size".
- Daily Mail