I was raised in America where we are told that we can 'have it all'. If you don't order the large portion, people wonder what is wrong with you.
I'm not blaming my situation on these factors, but I do think they are important contributors.
I think if 'order the large' is an option, you're already ordering from a takeaway/in a 'junk food' place.
1. It's ok to eat some "junk food" as part of your diet. That's not going to automatically kill your diet, if you still control calories. It might not be as satiating as eating other foods though, so it's worth experimenting with this. (personally I would still eat "junk food" on my diet and I enjoyed the extra flexibility).
2. Just cause you're eating junk food, since calories is what ultimately matters, it's still worth not ordering the large. Saying " well it's junk food I'm already lost" and then eating a larger portion is just making the sustain much worse than it needs to be.
3. Similarly, just cause it's a junk food place, doesn't mean there aren't better or worse options. I love McDonald's, and still eat there even when dieting to lose weight. But I'll eat only chicken nuggets, and skip the fries. Not nearly as good or healthy as eating other sources of protein, but definitely better than including an extra 500 calories of non satiating carbs.
I’ve been thinking about giving all but one serving away to friends each time I bake, but your comment reminded me that even one serving a week is likely more than I need.
I honestly only say this in case it helps, and I'm firmly on the upper end of 'healthy' UK BMI so not in any health freak position of judgement by a long shot, but I last had a slice of cake months ago, so the casual 'just an extra slice of cake every week' really stands out to me.
(On a complete tangent I don't at all think regular cake eating is incompatible with a healthy diet, but I do think that in 2022 it's highly inversely correlated, and that going against that almost requires baking your own.)