Balzac wrote a lot, quickly. If he had lived long enough to hear about the way Flaubert agonized over each sentence, he would have laughed, and then written a jolly story about a writer who ruins his life by agonizing over each sentence (no, not a writer, a painter - come back tomorrow).
Balzac's books sprawled as he aged. Henry James called Tolstoy's novels "loose, baggy monsters," which I have never understood, but that phrase is a good fit for A Harlot High and Low (1839-47) or Cousin Bette (1846). I find this frustrating sometimes, but Balzac was not interested in perfection. In the 1840s, in his 40s, Balzac seems to me to be completely confident in his methods and manners. He has loosened the stays of his corset, so to speak - see left, Rodin's portrait of Balzac, to get a sense of the work that corset would have had to do. He's comfortable, hopped up on strong coffee, and having a great time.
For example, look at this description, from Cousin Pons (1847), of a seedy attorney's office:
"The drain into which the household slops were discharged added its quota of nauseous odours to the stairway, whose ceiling was everywhere decorated with arabesques - such weird ones! - traced in candle-smoke... She [the servant] was unhealthily corpulent and wore an appalling dress of cheap printed cotton, with a Madras scarf tied round her head; her hair was still in curl-papers made of printed forms which her master received gratis; from her ears hung something resembling gold carriage-wheels... Monsieur Fraisier, a shrivelled and sickly looking little man with a red face covered with spots which spoke of impurities in the blood, and who moreover was constantly sratching his right arm, and whose wig, pushed far back on his head, incompletely concealed a sinister-looking, brick-coloured cranium..." Ch. 18
Why does Balzac, more than halfway through the novel, spend three or four pages describing the hideous M. Fraisier and his greasy office? Because it's so much fun.
Cousin Pons circles around a hack musician who has devoted his life to two things: collecting art and antiques, and sponging dinners off his relatives (one of the great pleasures in his life is the moment the lid is lifted from a covered dish). When he becomes ill, everyone he knows turns into a vulture, circling the art collection. Almost everyone - not his roommate and one true friend, the pathetic Schmucke. The scheming for the inheritance is one plot strand; the sorrows of Schmucke are the other. Both are quite good, one funny, one sad.
It's a bit of a surprise when, a third of the way into the fairly long Cousin Bette, Balzac declares that he has finally set the stage and can now begin the real story. It's even more surprising to read the same announcement ("And here begins the drama...") at the end of chapter 17 of Cousin Pons, about 60% of the way through the book. There's some evidence that Balzac started with just the art collection plot and came up with the friendship plot along the way. This is a relaxed approach to novel writing that would not work for a writer with a less abundant imagination than Balzac's.
Here's a bonus quotation from Pons, just for literature, and possibly anthropology, professors. One of the schemers has visited a fortune teller, allowing Balzac the opportunity to insert a long, long digression on the "occult sciences":
"Just now, when so many professorial chairs are being set up in Paris - chairs in Slavonic, in Manchurian studies, and in literatures so unprofessable as those of the North; chairs which, instead of offering instruction, stand in need of it themselves; chairs whose titular holders eternally grind out articles on Shakespeare or the sixteenth century - is it not a matter of surprise that, under the name of anthropology, the teaching of occult philosophy, one of the glories of the old-time university, has not been restored?" Ch. 13.
Translations by Herbert Hunt, Penguin Classics edition.