We've all met them. We ask politely about their holidays and they pat their stomachs before announcing: "I put on 5kgs in two weeks."
In other words, they had a good time. Too good a time! But keeping fit on holiday isn't for everyone - even the experts.
"Have fun," urges Nathan Feld, managing director of Melbourne-based Voyager Travel Corporation. "Going on holiday is definitely not the time to diet! As daily work routines decrease, food intake increases," he explains.
"Breakfast may be just cereal on work days - but on holiday it's cereal, bacon and eggs plus a croissant. Lunch may be a sandwich on wholegrain bread with no butter - but on holiday it may be noodles or gourmet sandwiches washed down with beer.
"Dinner is definitely lavish, accompanied by wine or beer."
And then there's morning and afternoon tea - an essential part of any holiday.
But the worst places for intake are cruises, says Feld. "Food is non-stop from early morning until midnight. With increased consumption there's also decreased exercise."
Travel industry opponents of holiday fitness programs say obsessions about staying in a shape can ruin dream holidays.
So, they advise, relax. Without making a pig of yourself, enjoy the food - particularly in places renowned for their cuisines - and savour a few sugary tropical cocktails decorated with umbrellas. Worry about shedding extra kilos after you arrive home.
But this easygoing attitude isn't for everyone. Some customers double-check that hotels have gyms, reveals David Goldman, director and general manager of Sydney's Goldman Travel.
Others have "all sorts of fitness regimes to keep them healthy". Then there are those, adds Goldman, who "are happy to kick back and enjoy themselves - and worry about weight gain after they get home".
Some big resorts have jogging tracks through their landscaped gardens - or, at city hotels, supply giveaway maps of jogging routes through nearby parks.
Most lodgings now have gyms - and, at hotels used by both business and leisure travellers, it's sensible to avoid the morning rush hour when health clubs are busiest.
Many fitness fanatics do a pre-determined number of lengths in pool each morning - while, at beach resorts, others go power-walking or jogging at the ocean's edge.
An alternative, if there's no gym or pool, is exercising in your room - doing push-ups, sit-ups, squats or a combination. Some travellers pack a skipping rope to use on the resort's lawn.
A massage at the resort's spa, with wonderful aromas wafting against a background of New Age music, may be relaxing and enjoyable - but it doesn't count as exercise.
Remember to take along running shoes and exercise clothing, says Richelle Kelly, owner of Suncity Travel on Queensland's Sunshine Coast, a Travelscene American Express affiliate. She believes the best way to keep fit without lessening enjoyment is to make "small changes to the way you travel".
One technique is to "try booking an active holiday: snowboarding in Japan, cycling in Italy, walking in New Zealand, surfing in Hawaii. There are so many options.
"Also, find every opportunity to work off calories: dance the night away in a Rio de Janeiro salsa bar, sign up for sea-kayaking in Thailand, take on-foot shopping sprees in Dubai's malls."
Kelly says people shouldn't forget the obvious - such as using stairs where possible, rather than escalators or lifts, in hotels and airports.
She also advises "doing your sightseeing on foot" for "a completely different perspective to looking out a bus window. You get a feel for a city: interacting with locals, tasting interesting foods - things you miss if you just drive by".
Meals? "There are easy rules that won't have you feeling left out," adds Kelly. "Don't start your meal with bread. Do you really need three-course meals every night? Beware of the dreaded buffet. Just because it's 'all you can eat' that doesn't mean you should. Don't go back for seconds."
And, if you have no willpower, she recommends avoiding buffets altogether. Lisa Ferrari, general manager of brands at travel.com.au, echoes Kelly's warning about over-indulging at buffets. "Limit yourself to one buffet meal a day," she suggests. "Remember, you'll bring any 'excess baggage' home with you.
"Exercise before you go on holiday - and try to stick to this routine while you're away. "Watch out for cream-based pina coladas. They may taste good but they won't make you feel good after you've returned home and it's time to wear your bikini again.
"People are tempted to drink more on holidays," adds Ferrari. "Swap beer and cocktails for lower-calorie drinks - and be sure to drink plenty of water."
Whether you make an effort to keep fit or you don't, the experts agree that you shouldn't forget your main objective: enjoying a hard-earned holiday.