Use the shell to naively insert 1gb of data:

var s = (new Array(1048576)).join('x');
db.test.drop();
for (i=0; i<1000;i++) db.test.insert({s:s})

While this is running watch the vmem and rss of the shell (eg. in top hit 'm' to sort by memory and 's0.3' to get fast update) — resident will grow to 1gb.

• Running gc() doesn't help
• Only affects write commands, does not happen if legacy writes are forced, so highly likely to be in the shell's Bulk API.
• Seems to affect all shells since 2.6 (write commands), though worse now on mozjs
• Same for ordered true and false
• Same for single inserts and multi inserts

• Variants which also exhibit the problem:

  var foo = null
  for (i=0; i<1000;i++) foo = db.test.insert({s:s})
  foo = null
  gc()
  

  var foo = []
  for (i=0; i<1000;i++) foo.push(db.test.insert({s:s}))
  foo = null
  gc()