@DeborahKerr @Parksinyork @Kitchenbattle
Thank you for the responses. I was playing devil's advocate a little bit in that I do actually have better systems for decluttering and keeping the unwanted stuff from coming in in the first place now than I used to, although it's not perfect. But I still think it's very interesting to compare the way we naturally lean because I do catch myself falling into old habits or ways of thinking if I'm not careful. Some of this reflects things I've learnt about myself or from going through some of the decluttering "systems" (Marie Kondo taught me a lot about the importance of getting things out ASAP).
I think the overall patterns from your replies seem to be - people who are naturally tidier think and plan more before buying things, so don't tend to buy things impulsively like food which "seems like a good deal" or seems like a healthy/interesting thing to try which then never gets used up.
People who have too much stuff are perhaps more prone to spotting a bargain or a shiny thing and thinking ooh I want that, without considering how it will fit with their existing things or where they will keep it or whether they have space.
People who are naturally tidier just do things straight away - like returning an unwanted/dysfunctional item to the shop, or listing things online or taking things to the charity shop. They are also less bothered by putting things in the bin, in general.
People who have too much stuff may have intentions to do these things but (IME) the items lie around for ages gathering dust or even worse, getting mixed back into the general chaos and having to be sorted out again endlessly. There is also a common theme of wanting to find the most "optimal" way of getting rid of something. I will never forget the shame I felt when I sold a pair of PJs on vinted which had been in my cupboard for literally years. They still looked good condition, but I hadn't put them on a child in years and hence I did not realise until the buyer let me know that the elastic in the waistband has totally perished! I gave her a refund even though she said she didn't want one, but that definitely pushed me into giving myself a deadline for online sales or I just charity shop the items or if all else fails, they go into a clothing donation bin.
And yes, good observation that I do tend to spend too much time on MN rather than doing useful things 
(I don't mind this at all, it's true, although I am much better than I used to be.)
It did make a HUGE difference to me when I stopped prioritising getting rid of things in "the most optimal way" (whether money, environmental impact, "waste" etc) and started prioritising whatever actually gets the item out of the house ASAP. So would 100% recommend this, OP.
Then lastly with the McDonalds' and party bag tat - I am much better at just weeding this stuff out ASAP when I see it and they have lost interest now - my problem before was that I couldn't whisk it away immediately because it would upset the children who usually delight in this stuff of course. And then once the initial novelty value of the toy had passed, I was not classifying it as junk to be got rid of, I valued it equally as all their other stuff because it was one of their possessions, and I feel strongly that it is important not to throw out children's stuff behind their backs (this is also something I have flexed on - I now think under a certain age, it's OK and even a kindness to do this, but once they get older, I do involve them in decision making about what to keep.)
DS1 and DS3 are unfortunately goblin hoarders like me, although DS1 got better about it as he got older and is almost minimalist now. DS2 is hilariously ruthless and doesn't care, mainly because he actually appreciates the space/clean areas.
DH was more ruthless than me when we first got together and he is more likely to "tidy" (which he does by shoving everything into whatever space exists anywhere in a room, and wantonly throwing out everything that looks vaguely like rubbish, including some things which are not). I am much more meticulous and detailed, but this means I am very slow and it takes more energy for me to clean up/declutter. I can look at a pile of "rubbish", and pick things out of it that I actually wanted to keep. And I don't like putting things in every random space, I want to put things away properly, which slows me down if there isn't a proper place for the thing. So I am getting there slowly especially with Dana K White (she has the A Slob Comes Clean podcast). Her technique is good for anyone who struggles to finish tasks, because it's little by little and doesn't make a bigger mess in the process.
I actually think we have reached a point now where I reject a lot of my previous borderline-hoarding tendencies and DH has more of the "I need to keep it in case it comes in useful later" and I am itching to go through all his stuff and decimate it. But I have learnt that it genuinely is more helpful for me to focus 100% on my own stuff, and the rest of the household follows.