A blocksize decrease is a soft fork because old nodes won't reject smaller blocks as if they were invalid -- when they see a small block they think "that's fine, there's no rule that says blocks have to be huge, they're just allowed to be"

A soft fork is a new restriction on bitcoin blocks or their contents, done in such a way that old nodes still see all the new block contents as perfectly valid

A hard fork removes a restriction on bitcoin blocks or their contents, done in such a way that old nodes see the new block contents as invalid because they break a rule that those nodes still enforce

It's not an intentional attack (except when I do it I suppose, because I explicitly intend it as an attack) but it does dump jpegs on bitcoin's blockchain so that it is harder to run a node. I run a node because I want bitcoin to be the best money it can be. To the extent that it is also a storehouse for monkey jpegs my motivation to participate falls a bit, because to me that "storing other people's random files" is a poor reason to run a node. I don't mind storing other people's transactions, even when it's somewhat hard on my machine, because doing so helps me know that my coins are valid. I do mind storing their nfts because there is no benefit to me.