nix gives No space left on device even though /nix has lots

I'm getting No space left on device when building (dependencies of) a nix project, even though both /nix and /tmp have >10G of space. I saw somewhere it might be using $XDG_RUNTIME_DIR (/run/user/1000) which is just 1.6G, but how would I increase the size of that / tell nix not to use it?

(This is on Ubuntu, not NixOS.)

EDIT: Fixed by export TMPDIR=/tmp (where /tmp has lots of space) and nix-shell --impure …. It seems nix builds stuff in $XDG_RUNTIME_DIR (a tmpfs) if TMPDIR isn't set, but will use TMPDIR if set, so that worked around it.