In the past we just ate less all round. Smaller portions of food, not eating between meals.
My parents were not impoverished in any way but, for example, if we had beans on toast for breakfast one standard sized tin of beans was considered as sufficient for four portions. Tinned fruit for pudding was, again, a standard tin divided by 4. Sweets were limited. My pocket money would not stretch to more than one bar of chocolate a week.
We didn't have biscuits or crisps or anything else between meals, there was no squash in the house and we drank water. This was the normal eating pattern in our household and not that different in any other one I visited.
The problem isn't sugar, or fat, or meat or any other single food stuff. The problem is that nowadays, all our food portions are too big. If DH and I have beans on toast, it is a standard tin between two, the same on the rare occasions we have tinned fruit. We spread the butter more generously than when we were children, have butter on potatoes. All these things add up and to children these large portins are the norm.