I've had my ZZ plant for almost two years now and I'm still puzzled by how slow it grows

I've had my ZZ plant for almost two years now and I'm still puzzled by how slow it grows. Everyone says they're bulletproof, and mine definitely survives my neglect, but I expected faster progress by now. I water maybe once a month in my cold climate and it just... exists contentedly in the corner. Is that normal, or am I missing something about light or humidity? I'm not complaining—low maintenance is exactly what I need—but I'm curious if anyone else's Zamioculcas zamiifolia takes this long to fill out.