Obese people may have found a good excuse for not going the extra mile, thanks to scientists.
It seems that the heavier you are, the greater your brain perceives distances to be.
Tests showed that a person weighing 23 stone sees objects as being twice as far away as someone tipping the scales at a trim nine stone.
The effect also applies to hill gradients. For a heavier individual, a hill will appear steeper than it does to a slim neighbour - another reason for taking the lift instead of the stairs.
Scientists believe the strange phenomenon is a survival mechanism inherited from our distant past.
It helps us evaluate testing situations in a flash, without having to think about them.
But it may also work against efforts to persuade overweight people to be more active and get off the couch,
Psychologist Dr Jessica Witt, from Colorado State University in the US, said: "Effort and performance influences perception.
"People who weigh more than others see distances as farther and hills as steeper, the idea being that if you have to carry this extra load that also impacts your perception.
"It is conscious perception of the world, but it's not based on conscious perception of the body or feelings of laziness .. And yes, it cannot be controlled, it is out of your hands.
"You can't will yourself to see that target as closer or that hill as less steep."
In one study, Dr Witt asked 66 randomly recruited people of different shapes and sizes to estimate how far away a traffic cone placed on a road was.
The cone was actually 25 metres away. But those who weighed nine stone estimated its distance as 15 metres, while volunteers weighing 23 stone judged it to be 30 metres from them.
Heaviness appeared to be the influencing factor, whether the weight be from our own bodies or a load being carried.
"If you find yourself out hiking with a heavy backpack, hills are going to look steeper, distances are going to look farther, gaps across a river are going to look longer," said Dr Witt.
"It's this idea that if you are going to have to make more effort to ascend that hill because you are carrying this heavy backpack, the hill is going to look different.
"You are not seeing that hill as it is. You are seeing the world in your ability to act. Your ability is declined because you are having to carry this rucksack."
In other words, journeys and hills appear more or less daunting depending on how much weight you are carrying. Dr Witt outlined the research at the annual meeting of the American Association for the Advancement of Science, taking place in Washington DC.
She was asked if one solution for obese people might be to wear telescopic glasses.
Dr Witt replied: "Yes. I would predict that. Give people an easier tasks to start with. Training on an easier target is a good idea."
In a related computer-based study, volunteers playing a virtual tennis game perceived balls as travelling more slowly when they were given larger racquets.
"We make you good by giving you a really big racquet or bad by giving you a really small racquet," said Dr Witt.
To measure judgement of hill steepness, volunteers were asked to rotate a line on a card until it matched what they perceived the gradient to be.
"Most people tended to over-estimate steepness. But the effect was greater when participants were overweight, or tired. Dr Witt agreed that the perception changes could undermine attempts to tackle obesity.
"We think that these perceptual biases can create a vicious cycle for people with obesity where they see the world as impossible to navigate," she said.
"They will be less likely to choose to be active."