I am overweight. I come from a family where on both sides there are very slim members who eat large portions.
I used to be one of them!
Too late I realised it was after giving birth this seemed to change for the female members. Regardless of age of mother when having first child so not a 'middle age' thing either.
I'd love to know if there's any scientific evidence of pregnancy/childbirth altering metabolism. Because there's also female relatives that are childless and they've remained slim.
I never used to need to worry about portion size. I lost the pregnancy weight just a few months after having dd and was back to my previous size 6/8 but about 2 years after I noticed I'd really started gaining even though I was eating the same and if anything activity was increased!
I reduced portions anyway and added an extra gym visit - was still gaining weight.
long story short I ended up at a size 18 and went ww and discovered my portion sizes were too big and I did massively reduce them and changed some high cal foods for lower cal ones, but I didn't lose loads.
Few years ago I decided I was going to eat what I wanted when I wanted BUT listen to my body, eat what it was asking for and stop when full. I've since lost a little more weight but am still overweight but made my peace with it - it's at this point partly due to meds in any case which are KNOWN to slow metabolism.
In my family I've honestly when staying with people observed what and how much they eat and there are definitely overweight members that barely eat, and slim ones that not only eat big portions but high calorie density foods too. In addition the slim members are not always the most active ones either.
Yes, generally speaking weight = calories in vs calories out but I do think there's other factors too.