Unexpected growth in mdb database file

["Rene B. Elgaard" <rene.bach.elgaard@gmail.com>]

I have gone through an upgrade proces from OpenLDAP 2.4.21 to 2.4.40 and 
took the opportunity to switch from the bdb backend to mdb.

I am expiriencing an (in my eyes) unexpected growth of the mdb database 
file.

Two types of setup:
Setup 1: One master, one slave
Setup 2: Two masters MMR (read,write,syncrepl goes to one master, three 
slaves)

I see the growth in both setups on the master, who services read,write 
and syncrepl.
In setup 2 the master, who is "idle" and only syncrepl's with the other 
master, does not grow more than expected.

Example from setup 2:
Servicing master:
-rw-------   1 openldap openldap 4294963200 Nov 26 10:18 data.mdb

Non-servicing master:
-rw-------   1 openldap openldap 271347712 Nov 26 10:18 data.mdb

Quite a difference !

A slapcat on the servicing master shows that I have no where near 4 Gb 
of data, here from my nightly backup:
-rw-------   1 openldap openldap 73389431 Nov 25 23:46 
20141125234504_slapd-dump.ldif

I have the same backend configuration on all masters:

database        mdb
maxsize         8589934592
suffix          "c=<country>"
rootdn          "cn=Manager,o=<some-org>,c=<country>"
rootpw          <some-pw>
directory       <some-path>
index   objectClass,entryCSN,entryUUID  eq

No overlays involved.

Operating system is Solaris 10.
OpenLDAP compiled with Oracles compiler, compiler flags are:
export CC="cc"
export CFLAGS="-m64 -fast -D_AVL_H"

Unsure how to proceed. Should I file an ITS (looks somewhat like 7974 
though) ?
I may also have missed something entirely...
Happy to provide further info.

/Rene