If someone is eating exactly enough calories to covere TDEE (total daily energy expenditure) and is thus at maintenance, given that 70 calories given in the most efficient form (carbohydrate) would mean little energy lost to processing, and 1 gram of fat is 9 calories, that's 7.7 grams of fat added every day. In 3 months, that's ~700 grams of fat added. 1 year, ~2.8 kilos. In 5 years, over 14 kilos.

That rough calculation won't be highly accurate for a number of reasons, but you could halve it and it still is very bad, thus it goes to show how easily fat can accumulate once maintenance calories are breached. Take care of the pennies and the pounds add up. Same goes for calories.