Jack-in-office is a great man.

The saying is ironic: a minor official, once given even limited authority, may behave as though he were highly important. “Great” is exaggeration, pointing to inflated self-regard rather than genuine stature.

It disparagingly criticizes the way a small post can magnify someone’s manner, making them pompous, domineering, or overly conscious of rank. The target is pretension attached to office, not necessarily competence.

Use it as a disparaging, ironic comment on someone who becomes self-important after receiving a modest role or little authority. It can criticize bureaucratic pomposity, but its insulting tone may sound personal and contemptuous when directed at an individual.