Amelia's older brother is a very vain, very fat, very self-important officer in the East India Company. Although he is at first able to escape being married to Becky, eventually she re-ensnares him and – perhaps – kills him off for his life insurance money.
Jos is pretty much a walking cartoon. Every one of his qualities is exaggerated to be make the reader laugh, and every decision he makes or action he undertakes just shows us all the more how ridiculous he is. Although the humor is fairly broad, it is cleverly layered. Plus it helps the fat jokes go down easier if we know that Thackeray often referred to himself as a fat, overdressed man.
Take for instance Jos's size. There's no beating around the bush here – the dude is hugely fat. Some of the laughs are built just around this fact, kind of a 19th-century version of Chris Farley type fat-guy-falls-down humor. Remember, for example, the painting of Jos sitting on top of an elephant shown at the Sedley auction. Some wag in the audience asks the auctioneer which is which. Hardy har!
If George is an externalized gentleman, Rawdon a genetic gentleman, and Dobbin a psychological gentleman, then Jos is a parody of a gentleman. He has many of the characteristics we have observed in these three more realistically described men, but within each category there is a built-in joke that rests on his fatness. There is the materialistic angle: he is obsessed with clothes but for some reason has them made far too tight, so they bulge and stretch in unattractive ways. There is the vanity angle: he imagines himself to be extremely attractive to the ladies and generally behaves like a Don Juan...except when actually in the company of women. There is the macho angle: when he goes to Brussels, Jos grows a giant military-style mustache...and immediately shaves it off in a panic that he might be thought to be a soldier. And at the same time, there is the generosity angle: he is happy to give his destitute parents and sister money...but his desires to see them end whenever his comfort and easy access to food are disturbed.
So whom exactly are we laughing at here? Jos is funny, but is the joke on him? Or is he just a funhouse mirror image of the novel's more well-rounded men?