mongod memory usage much larger than cache_size for wiredtiger b-tree

Memory usage by mongod is much larger than the cache_size setting for the WT b-tree. This is also a problem for the WT LSM. When using cache_size=8G and then running iibench I see that mongod vsz and rss are larger than 60G for the WT b-tree:
root 1919 111 44.3 66988456 65824412 ? Sl Dec17 1378:17 bin.tcm/mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig=cache_size=8G,direct_io=[data]

And larger than 30G for the WT LSM:
root 20861 225 22.0 40058892 32678208 ? Sl Dec17 2783:38 bin.tcm/mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig=cache_size=8G,direct_io=[data] --wiredTigerIndexConfig=type=lsm

From /proc/$pid/smaps for the WT b-tree there is one huge allocation for the heap:
02c10000-febde6000 rw-p 00000000 00:00 0 [heap]
Size: 66733912 kB
Rss: 65805752 kB
Pss: 65805752 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 65805752 kB
Referenced: 65805752 kB
Anonymous: 65805752 kB
AnonHugePages: 0 kB
Swap: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB

And for the WT LSM the heap allocation isn't as huge (less than 8G):

02c10000-1c46fe000 rw-p 00000000 00:00 0 [heap]
Size: 7367608 kB
Rss: 5496284 kB
Pss: 5496284 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 5496284 kB
Referenced: 5496284 kB
Anonymous: 5496284 kB
AnonHugePages: 0 kB
Swap: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB