If they have any sense they'll not try to find out what's on it and send it straight to whatever electronics recycling is available.
Sticking a USB device of unknown provenance into your computer is just asking for trouble. (When you think about it, we even take a risk every time we buy one.)
Sure, you know it's harmless, but they don't know that, even if you tell them. Who are you? You're just someone who used to live in their house. As far as they know, you might be a freak who gets a kick out of leaving dodgy devices around for people to find.
Maybe if you use a file system that supports compression, e.g. btrfs, bcachefs, F2FS, squashfs, or EROFS. Of course, you'd need to add a separate FAT32 EFI System Partition for the bootloader, not sure how to do that.
I love netboot.xyz. I use it all the time when setting up VPS systems. A lot of KVM-based VPSes have iPXE as a boot option so you can chainload directly into netboot without having to use an ISO.
I prefer installing the OS myself over using any images provided by the provider, so that I know exactly how it was set up.
Netboot.xyz has tools to build your own custom version of it too, with your own options. Useful if you want to host it on an internal server. It's essentially just a set of iPXE scripts.
