In Joseph Conrad’s novella Heart of Darkness, Charles Marlow has been around. He’s an old hand at navigating boats through difficult terrain, and there’s little he hasn’t seen. He is, in short, a cynic. He’s experienced at transporting the minerals and other goods, including ivory, that are the stock-in-trade for the British and French companies colonizing Africa. He was, as described by the introduction to the story’s narrator,
“. . . the only man of us who still ‘followed the sea.’ The worst that could be said of him was that he did not represent his class. He was a seaman, but he was a wanderer, too . . .”
As is often the case when an old-timer like Marlow leaves the environment in which he is most comfortable and enters the world of corporate lawyers, accountants and myriad “white collar” lackeys, he holds those above him in the corporate hierarchy in disdain. It is in this context that, when summoned to company headquarters, Marlow’s attitude towards this particular world are expressed most forcefully:
“I had no difficulty finding the Company’s offices. It was the biggest thing in this town, and everybody I met was full of it. They were going to run an oversea empire, and make no end of coin by trade.”
Heart of Darkness is an early and scathing indictment of imperialism. Marlow’s observations as he navigates down the long river deeper into the jungle include the morally questionable role of Europeans in this far-off section of the earth:
“The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much.”
Marlow, and those like him, see up close the implications of the decisions made thousands of miles away in cosmopolitan capitals like London. Like the soldiers who resent being told how fight a war by the politicians in suits back home, Conrad’s protagonist views those at the company offices with a kind of contempt born of his years of experience. When he refers to the employees in the company offices as being “full of it,” he is using an oft-quoted phrase indicating that those who claim to know better than others frequently do not. The sense among decision-makers that they are acting in the best interests of humanity frequently are, in Conrad/Marlow’s eyes, merely pursuing profits at the expense of foreign peoples they’ll never set sight on themselves.
"Full of it" implies hubris, pride, self-importance and so forth. The passage states that the company office is "the biggest thing in town", which we might interpret literally and figuratively; it is both the largest building and the center of attention. Everyone working there is puffed up with a sense of grandeur - after all, they're going to create their very own empire and become rich in the process. "Full of it" is usually used as a euphemism for words that we, or the writer, choose not to say, and to make a tongue-in-cheek suggestion that the person or things that are "full of it" are arrogant or not very easy to relate to.