I didn't watch it, I'd have felt ill! I do watch Secret Eaters though, a guilty pleasure is shouting at the people
Apart from denial, one of the biggest problems seems to be that they (meaning really extremely overweight people in general) have grown up having no idea what 'normal' is. A normal meal, normal-sized portions, etc...
Last week's couple had cameras in their house for 5 days, and over that period at no time did either of them prepare or even open a pack of what you would call a meal! The amounts they ate in the course of a day were truly staggering (a snack for the bloke was a meat pie between 2 slices of bread

).
But so many people live like that, no set mealtimes, no proper meals, no idea of what constitutes a normal diet, just constant shoving food in - and doing the same to their children. These particular two had a little girl, a toddler, but she wasn't shown really in the programme - I really hope they'll have learnt enough to give her a better chance.