Having approached Stephen's consciousness with the word "Chrysostomos," the narrative enters its orbit again by saying that Mulligan's "plump shadowed face and sullen oval jowl recalled a prelate, patron of arts in the middle ages." Thornton cites good evidence from a 1912 essay by Joyce to suppose that he had in mind Pope Alexander VI (Roderigo Borgia, father of Cesare and Lucrezia), a notoriously corrupt Renaissance pontiff and famous patron of the arts.
The word "plump" may have conveyed some of Mulligan's consciousness of himself in the novel's first sentence, but when the narrative returns to it (three more times in Telemachus), it conveys Stephen Dedalus' distrust of his companion. Stephen thinks of “the wellfed voice beside him” with the same distaste that he later feels for the “moneyed voices” of students at a British school.
Like Joyce himself at the same age, Stephen has recently returned from a self-imposed exile in Paris where he had little to eat. (Joyce complained often and pitifully to his mother, who sent him money orders when she could.) From this perspective, Mulligan's "plump" appearance evokes Stephen's lean and hungry resentment of his well-fed benefactor.
Church patrons like Alexander were often imperious to the painters, sculptors, architects, and musicians whose work they facilitated, and the artists inevitably had to balance their own artistic visions against the aesthetic and ideological requirements of their patrons. Stephen has good reason to feel that Mulligan does not have his best artistic interests at heart—just as he has good reason to feel that Mulligan scorns him for his physical hunger (a notable failure of compassion in a man who purports to be charitably supporting him).