Mongostat display size / res I would like to have your opinion and of course an explanation on the following.

My configuration : 


mongodb version 3.4.4
1 replicaset with three identical servers( 4 cpu / 32 Go RAM under linux ubuntu v16)
1 primary and two secondaries.
No delay in replication every thing is up ta date.
The engine is WiredTigger.


During insertion or update time I have the following:


Primary : vsize : 3.73 G/ res : 3.02 G
1st secondary  : vsize : 3.28 G / res : 2.63 G
2nd secondary : vsize : 5.66 G / res : 5.02G 


I am wondering why is there this difference between the vsize/res of the 2nd secondary and the 1st one?

I was guessing that I should see some kind of equality between them because they both support the same informations from the primary.

Any help or information will bee appreciated.
Has those servers been on that order during their uptime? It looks like that current 2nd secondary has been primary earlier during time what this replica set has been up. Because mongod memory consumption grow (until it reach it's maximum) when it reads data/indexes from disk. (and of course when you insert documents)

At start, mongod process eats only little memory, but during time when it serve clients, it will consume all available free memory, eventually! Because secodary nodes don't (normally) server/receive queries, their memory consumption grows much slower than primary.