How can businesses create large prints from small images (even before the advent of AI)?

I'm currently looking to buy a printed canvas of an artwork called 'Cherry and Maple Trees' (by Sakai Hōitsu).

I can find online (mostly on Etsy) very large canvas reproducing this work - up to 90x40 inches (or 228x100 cm). And apparently they're prints of good quality (costumers seem happy).

The thing that shocks me is that the biggest image I can find of this artwork - image made available by the Museum itself hosting it (it's public domain) - is very small in size (less than 3Mb).

So my question is: how can an Etsy seller print such a large canvas from such a small image, while preserving the image's quality? Is it all AI? If so, how did they do it before? Do they get access to a secret stash of high-resolutions pictures (even for the ones already legally free like my artwork)?

Thank you in advance to help me understand :)