Use the actual allocated size for the blobs

Change-Id: Icdbb5267d831482a985ed628c1b2d97d29c45131
Reviewed-on: http://review.couchbase.org/37430
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>