Lots of factors.
But mainly eating and drinking too much of the wrong things, and having generally sedentary lifestyles. Calories were simply harder to come by.
My grandparents indeed lived through the war and ate plainly, but then they ate like that for the rest of their lives too.
Their daily meals world be something like this, although they smoked a lot too:
Breakfast; small bowl of cornflakes or one round of toast, tea
Lunch: plain sandwiches, with an apple or orange, more tea
Dinner: meat and two veg with boiled potatoes / homemade chips and egg / hotpot / ham or egg salad in the summer. More tea. Occasionally just boiled eggs on toast.
No desserts, very few treats, they didn't really drink anything except tea.
Treats would have been the odd shared portion of fish and chips on a day out, two Marie biscuits with butter on a Saturday afternoon, a few chocolate caramels on a Saturday night and a sherry/whiskey and dry only on a very special occasion.
They walked to work, had manual jobs, looked after their gardens, did a lot of manual housework and DIY.