Consuming an extra glass of wine or half a bag of crisps a day can cause a person to gain almost 5kg (11lb) over a year, experts have said.
Over-eating by 100 calories a day can lead to the weight gain, the World Cancer Research Fund said.
To mark Cancer Prevention Week, the charity has launched the "100 Calorie Challenge" to encourage people to make small dietary changes which could cut their risk of developing cancer.
Being overweight or obese is one of the leading cancer risk factors, a charity spokesman said.
He said that in the UK, 18% of cancer cases are linked to people being overweight.
"Anyone who's ever tried dieting will know how difficult it is to lose weight, so we're proposing the 100 Calorie Challenge so people can avoid putting on that extra weight in the first place," said Kate Mendoza, head of health information at the charity.
"Cutting 100 calories from your daily diet is relatively simple - equal to cutting out one and a half digestive biscuits - but the cumulative effect of such a small daily amount could prevent nearly an extra stone in body weight over a year.
"This strategy of small changes could prevent additional weight gain and help reduce cancer risk. Small sustainable changes are easier for people to follow and better than larger ones that can't be maintained."
People who take part in the challenge will receive free daily emails giving advice on how to reduce food and drink intake by 100 calories.