Research conducted by the former medical adviser of Strathclyde Fire and Rescue service revealed most firefighters find their waistlines expanding after joining up – gaining up to 61lb in the most extreme case.
Dr Christopher Ide, now a consultant physician in occupational medicine for NHS Lanarkshire, warned chubbier crews are increasing risks to themselves and those they rescue.
Fat firefighters are more likely to suffer ill effects from intense heat, can struggle to squeeze through small spaces and can find they cannot carry victims down ladders that have a weight limit, he said.
Dr Ide said: "What particularly astounded me was the speed at which people went from being fairly svelte to being very fat, and how much additional weight they put on."
New recruits to Scotland's fire services have a medical examination when they enlist, and are re-examined further into their career if they apply for an LGV licence to drive appliances.
For his research, which is published in the latest Scottish Medical Journal, Dr Ide compared the weight and body mass index (BMI) measurements of 114 firefighters at enlistment with Strathclyde Fire and Rescue service, and their vital statistics at the time of their LGV medical.
There was an average of 3.7 years between the examinations and an average weight gain of 13lb.
Six of the firefighters were initially underweight, 58 had the optimal BMI and 50 were classed as overweight. By the time of their LGV checks, however, 73 of the firefighters were overweight and nine were obese.
Dr Ide notes that BMI calculations, which look at whether a person's weight is appropriate for their height, can misclassify the particularly muscular.
However, when asked if the changes could be attributed to the firefighters becoming more athletic, he said: "It would be nice to think these people had got a lot fitter because they were working for the fire service, but I would expect not."
He said his prime concern was firefighters struggling to cope with the heat on jobs and suffering effects ranging from exhaustion to sudden – potentially fatal – heat stroke. He added that there were also long-term health consequences of being overweight, such as type two diabetes.
Dr Ide said: "Now the age of retirement has been pushed up by five years, you have got five years longer for people to develop these various diseases, and all the time you have got the acute problems I refer to.
"I think the fire service needs to be more vigilant about keeping firemen in shape.
"It is not through lack of trying, though, I think. They have had a rolling programme of refurbishing fire stations and one of the things they did was include space for a gym equipped with various machines."
A spokesman for Strathclyde Fire and Rescue said: "For a number of years Strathclyde Fire and Rescue has ensured its firefighters are assessed using national UK testing standards.
"We take the health and fitness of our crews extremely seriously and have robust procedures in place to ensure every firefighter is fully capable of meeting the physical demands of the role. We ensure that while on duty our personnel have access to sport and other physical activity."
Strathclyde firefighters are tested every three years to check aerobic fitness and weight, and anyone outside the prescribed range is not allowed to continue active duties.