Yeah, it's really too small. Some programs will not let you choose what drive to install on. And you need to leave 20% of your drive with free space so it can operate properly. And if you start adding CC and mods, not to mention future packs and whatnot, that 256GB sounds like a lot, but it's really not.
After windows 10 and what not, you might have closer to 200GB, and then leave 50GB free and open to operate, that's down to 150. That's really not a lot of space.
There has been many people that felt that 256GB was enough, until they started gaming and ended up having to transfer to their bigger HDD cause it's not enough.
As a reference:
https://i.imgur.com/YHxSJ07.png
That's my 1TB SSD. Which isn't a full 1000GB, but ends up being 930GB, and I only have Sims 4 and required programs on my SSD. I'm using around 233GB, which would put a 256GB SSD in the "too full" zone, or even completely full, depending on what the size shows.
I haven't installed all my stuff back on my computer since my last round of upgrades, so I only have Sims 4 on my SSD.