Genetics, I guess. Although having said that, I'm a different body shape from my family – tall and lanky. My mother's family is quite dumpy, my dad's a mix of dumpy and wiry but short. I'm a bit of a mystery.
Also I don't drive, so (pre-lockdown/restrictions at least) walk quite a bit, even just to and from bus stops, around the tube network etc. I think driving as opposed to walking is one of the biggest factors in weight; friends and family members of mine who live in more rural areas (I'm in London) and drive all the time are pretty much all notably plumper than me.
And, while I think I eat good food portions, and don't avoid anything –I eat fat, sugar, carbs including white carbs – I am surprised when I hear or read about people eating a whole pack of biscuits etc. I genuinely only enjoy about two biscuits and after that I'll stop because I just don't fancy any more.
I exercise a bit – some yoga, some dance aerobics. Not madly though, maybe an hour or two of dance aerobics a week and about three yoga classes, not particularly sweaty ones.