She apparently eats two bowls of Sugar Puffs for breakfast, and up to five packets of crisps a day. So perhaps, like many women, she simply finds it hard to put on weight?
Some people can't help being skinny. It is not every woman's dream - and in fact, it can be deeply traumatic. As well as having to deal with verbal abuse ("Bag of bones! Skeleton! Stick insect!") they are more susceptible to health problems.
"Thin women need to be ultra-careful," advises Sian Porter, of the British Dietetic Association, the professional body for state-registered dieticians. "As soon as they get run down, they have few reserves to fight illnesses. They are prone to infections, skin problems, dull hair and eyes, osteoporosis and fertility difficulties."
There may be a medical reason for being underweight. "If people have had cancer, or are recovering from surgery, they can become very thin," says Ms Porter. "However, in many cases, being thin is simply a case of genetics. It does not have to be a problem if these women eat a healthy, balanced diet and maintain their weight."
Nutritionists use the Body Mass Index (BMI) to judge whether a person is underweight. "Weight in kilos is divided by height in metres squared," explains Ms Porter. "If the result is between 15 and 20, the person is deemed to be thin. If it is less than 15, their weight is life-threatening."
Seven per cent of British women are too thin, according to their BMI. Helena Fishlock-Lomax, 37, from Wellesbourne, Warwickshire, is among them. Her BMI is 17 (she's a size six, 7st 7lb and 5ft 6in tall). She founded the Size 8 Club in 1996 to offer support for slim women.
She says: "I feel very sorry for Posh Spice. She has good looking hair and eyes, which are a far better barometer of health than her waist size". She identifies with the Spice Girl's situation: "I find people are very rude to me. I call it `skinny bashing'. I'd never dream of telling someone they're fat.
She adds: "Dawn French and Jo Brand get positive press about their weight and make it possible for larger women to be accepted. But slimmer-than- average people get accused of being anorexic."
Ms Fishlock-Lomax recently published her own book, The Slenderella Syndrome - the Slim Woman's Survival Guide, which offers health tips, a clothes directory, and advice from other slim women.
It could be just what Victoria Beckham really, really wants.
For copies of `The Slenderella Syndrome' call 01789 473 355. The Size 8 Club: 01789 842 307
`THEY USED TO CALL ME STICK LEGS'
"I THINK Posh Spice should be left alone", says Karen Mercer, 26, a size eight beauty therapist from Westbrook in Warrington. "She's not too thin and obviously eats healthily. She's simply got herself back in shape after having a baby. She looks fantastic".
Karen understands what the Spice Girl is going through, as she is often taunted about her size. She joined The Size 8 Club two years ago, to help her to come to terms with her slim figure.
"I am 5'6" and weigh 7st 10lbs", she says. "At school, classmates would compare me to a hockey stick or a golf club and call me Minnie Mouse and `stick legs'. They'd tell me I was anorexic and accuse my parents of starving me.
"I became paranoid about the way I looked, especially my legs. I would always wear baggy clothes to make myself look bigger.
"Even now, shopping is a nightmare. The assistants direct me to the children's departments. Someone in Principles once suggested that I should `go and eat a nice big sandwich'. A lot of my clothes come from New Look's aged 9-15 range. I have just bought an M&S top for 13 to 14-year-olds.
"The communal changing rooms are the worst. Women stare and say: `Look at her underwear. Only anorexic people can wear things like that.' My best friend says I wear `whippet clothes'.
"Despite being so small, I eat a lot - a balanced diet with a little bit of everything, including chocolate and cakes. I cannot imagine going on a diet. It must be the worst thing ever.
"In terms of my health, being small can cause problems. I worry that I will find it difficult to have children without a caesarean, because my hips are too narrow.
"Also, I get run down very easily and catch every cold that is going around. In 1991, I contracted viral meningitis three times. The Meningitis Research Society told my mum that tiny people have very low immune systems. They suggested taking vitamins every day.
"Being slim can destroy your confidence, but meeting others through the Size 8 Club helps you to realise that you are normal. It still hurts when people make comments, but I am happy with myself now."