For 22-23 years of my life I've lived in Europe and then moved to Asia. I've been living here for 10+ years now, and I can't help but notice lots of cultural differences when it comes to weight. Can't speak for UK though as I'm not from there, but my country has plenty of overweight people too and it's quite noticeable to me after getting used to seeing slim people everywhere over here.
First thing that comes to mind is peer pressure. It's generally encouraged to comment on other people's weight as you may help someone notice and recognize a shift in their weight so that they can promptly deal with it. I was a UK size 10 (sometimes 12!) when I arrived here and I was surprised when my weight got commented on at my workplace for the 1st time. Later I considered myself lucky though as my overweight friend who had the same credentials as me struggled to find a job (and one of the recruiters hinted that that's because of her weight). Nowadays I see big companies employing plus-size people though in the name of "diversity".
However, I can't imagine commenting on someone's weight back in my home country, that'll be considered extremely rude (and you can see many people bashing OP here - even though it's just an anonymous post on some internet forum). Basically it's sort of a taboo subject, you say nothing or you say "love yourself the way you are, girl!" to be praised as kind - a striking difference with Japan where I live.
Secondly, people over here tend to have a completely different mindset when it comes to weight gain in pregnancy. Doctors actively encourage you not to gain over some limit they calculate for you and you are strongly recommended by everyone you know to lose weight within first 6 months after giving birth. Back in Europe it's usually not seen as something terrible if you still have a little bit of extra weight on you after giving birth 1-2-3 times. Your body has grown a new human, you deserve to love yourself whatever weight you are etc. Sometimes people would even comment that you don't look "motherly" enough if you slim down lol! Giving birth is generally seen as a legitimate excuse basically.
In Japan, things are very much different, almost everyone snaps back as fast as they can, I did my best to return to my pre-pregnancy smallish size 8 and most of the foreign women I've seen here have been doing their best too to return to their shape. Many times I've seen slim and toned Japanese mothers going out and about with their 3-4 kids - number of times you gave birth is never considered an excuse. Anyway, as I've said, it's a very different mindset.
The way I see it, it's the factors I mentioned above that mostly contribute to the topic OP's mentioned, not smoking or too many sugars in the food or whatever, but it's just my personal opinion and I could very much be wrong.