“Rosh katan”, literally “little head” is used to describe incompetent and/or passive-aggressive behavior. It usually involves following orders literally, without understanding their goal, or understanding, but actively wanting not to achieve it.
“Rosh gadol”, “big head” – is a way of working proactively, anticipating problems that might arise from orders that are not overly detailed, and following the spirit or the order. You really need to grok your task in order to act “rosh gadol”.
I asked a co-worker who happened to be an Israeli paratrooper captain in his previous life. He said that they also had a third expression – “rosh nitz-nitz” (I am not sure of the spelling here). “Nitz-nitz”, he told me, is a nickname for an ingenious little device that paratroopers use for identification at night. It’s a little infrared emitter invisible by unaided eye. But when looked at with infrared goggles at night, it allows helicopter pilots to find paratroopers who are ready for extraction.