It probable that the vast majority of that 1MB of memery per dormant thread is being consumed by an wildly over-generous per-thread stack allocation that will likely never be consumed. See Use more threads..

I've had 3000 active threads running (not doing much; incrementing a counter and displaying it at a particular location on a 200x100 console session, but more than sleeping), all in under 1GB.

Greedy in C terms, but not bad for 3000 independant interpreters.