Cap collection storage size well described well for MMAP, since WT compress the disk size, we should modify the documentation

http://docs.mongodb.org/manual/core/capped-collections/

We might have similar issues with similar vector of problem (size of the disk w/o compression)

see example below.

  "ns" : "sbtest.sbtest",
  "count" : 843881,
  "size" : 999,998,985,
  "avgObjSize" : 1185,
  "storageSize" : 219,582,464,
  "capped" : true,
  "max" : -1,
  "maxSize" : 1,000,000,000,
  "wiredTiger" : {