Considering that Mr Kim, an ensign, occasionally was in command, being an ensigt in Engineering must quite high in the ladder there.
Wasn't Harry suppose to be like gifted in the field?
Didn't he graduate at the top of his class or something?
Wasn't that implied in "Non Sequier"
And yet he didn't graduate as a Lt(jg) like Kirk, NuKirk, NuUhura etc. Kim is the Chief Operations Officer - as far as I can tell he didn't inherit the role from a senior officer. Each key position requires three officers (one for each shift) so Kim would be the main man with two people (most likely ensigns) lower than him in the chain of command but of equal or lower rank - it seems that helm and ops are staffed by officers (Helm = command; Ops = engineer) usually but occasionally by chief petty officers (O'Brien, DiFalco - although she was technically navigation rather than ops).
Engineering is a bit different because you have a lot of crewman carrying out standard maintenance and repairs at all times, supervised by officers or senior non-coms. On a ship the size of Voyager I can't imagine that they can have much more than half a dozen officers to supervise the various shifts and key positions overall. Vorik most likely filled the role of assistant chief engineer when he was on duty. Carey, as a more senior officer would be in charge of shifts when Torres wasn't on duty, which is why we rarely see him, with a different ensign or CPO as his number two.