I created a simple test to just insert into a 1mb capped collection
windows
with journaling
memory growth after inserting 2m items is around 4gb and if I let it continue it will use all available memory

without journaling
memory quickly stops growing no mater how long test runs.

on linux
with journaling
memory quickly stops growing no matter how long test is run.
without journaling
memory quickly stops growing no matter how long test is run.

I tried in 1.8.3 version with same problem.