Node.js (x64) runs out of memory despite using less than 2G

output.txt

% coffee test.coffee
MEM: rss 0.02 Gb, vsize 2.91 Gb, heapTotal 0.01 Gb, heapUsed 0.01 Gb
File no. 1
process: 2474ms
MEM: rss 0.24 Gb, vsize 3.11 Gb, heapTotal 0.23 Gb, heapUsed 0.22 Gb
File no. 2
process: 4509ms
MEM: rss 0.64 Gb, vsize 3.5 Gb, heapTotal 0.63 Gb, heapUsed 0.6 Gb
File no. 3
process: 3076ms
MEM: rss 0.76 Gb, vsize 3.62 Gb, heapTotal 0.75 Gb, heapUsed 0.72 Gb
File no. 4
process: 4127ms
MEM: rss 0.87 Gb, vsize 3.73 Gb, heapTotal 0.85 Gb, heapUsed 0.83 Gb
File no. 5
process: 3347ms
MEM: rss 0.98 Gb, vsize 3.84 Gb, heapTotal 0.97 Gb, heapUsed 0.95 Gb
File no. 6
FATAL ERROR: JS Allocation failed - process out of memory

test.coffee

index = 0
urls = {}

for file in [1 .. 20]
  ((m) -> G=1024*1024*1024; console.log("MEM: rss #{Math.round(m.rss/G*100)/100} Gb, vsize #{Math.round(m.vsize/G*100)/100} Gb, heapTotal #{Math.round(m.heapTotal/G*100)/100} Gb, heapUsed #{Math.round(m.heapUsed/G*100)/100} Gb"))(process.memoryUsage())

  console.log "File no. #{file}"

  console.time 'process'
  for i in [0 .. 1500000]
    url = "url#{index++}"
    urls[url] = { humans: 0, robots: 0 }
  console.timeEnd 'process'
  undefined