How does this compare to a lightweight Linux distro like Lubuntu?
Lubuntu is not really "lightweight" in my experience. A really light Linux install uses maybe 0.2 GB RAM at the desktop, which lets you do some very basic web browsing even on a 1 GB system.
I can get a fully functionable desktop with GUI apps, generic hardware support (i.e. not locked down to my hardware), support for dynamic modules/drivers/libraries, audio, 3d-accelerated video, and more in a 300 MiB footprint (with only basic iso image compression) and runnable with 128 MiB of RAM.
Then comes in the last part of your statement: "do some very basic web browsing." The system above works just fine with a browser featuring < 201x tech, with great CSS, JS, HTML support. But if I need to build and bundle the latest Firefox, Chrome, or whatever without manually stripping out a ton of features (beyond what is available via distro package managers), that footprint triples or quadruples in size and the memory requirements skyrocket.