Mongostat display size / res
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.