Tiberius, Enterprise was 10 times the size of Voyager, which means the job was probably ten times as hard and the staff under him (Yes Kim has staff under him, but not very many and all enlisted.) was tens times as large and might have included ensigns and lieutenants as well which then required a lieutenant commander to ensure the pecking order. Every blinking bulb on the operations LCARS is actually some guy in a room making sure some really important machine doesn't blow up, but when it does start to blow up, they send a poke through to Kim, or Data to let them know that shit is getting real..