Less than 2GB OS ISOs not fitting on 2GB USB drives

In non DD mode, Rufus will convert from one file system to another, which means that there's no guarantee that what occupies less than 2 GB of content with one file system will also occupy less than 2 GB in the other. There are many elements that can make files take more space when copied from one file system to another, such a cluster size, fragmentation and so on.

Also, when Windows reports that an ISO is 1.8 GB, it means that it occupies about 1.8 x 1024 x 1024 x 1024 bytes = 1932735284 bytes of disk space. That's because Windows uses the historical definition of a kilobyte which is 1024 bytes. But when a manufacturer says a flash drive is 2 GB, it means that it can accommodate up to 2 x 1000 x 1000 x 1000 = 2000000000 bytes, because drive manufacturers use their own smaller definition of a kilobyte in order to make their drives look larger to unsuspecting consumers. So, in "manufacturer units", there's a difference of about 0.068 GB rather than 0.2 GB between the size of your drive and the size of your ISO, which isn't that much.

As such, it isn't surprising that the content from your 1.8 GB ISO may not fit on a "2 GB" flash drive when written in ISO mode. To make it fit, you may have to select DD mode when Rufus prompts you.