Niri doesn't use 400MB by itself, that's the entire memory footprint of everything running. In comparison, OpenBox with all the utilities needed for wallet, ssh agent etc is in the 450MB range on my box. That's probably due X11 vs Wayland.
A minimal Niri functional environment is similar to IceWM in RAM usage. I used to run antiX in VMs.
A minimal Niri functional environment is similar to IceWM in RAM usage. I used to run antiX in VMs.