Actually, what happens here is usually that the publisher is willing to take more of a chance on a longer second novel. The longer a novel is, the more paper it takes to publish it, and paper is what costs the publisher big bucks.
Paper costs really do add up quickly. If you have a modest print run of 10,000 copies, and your novel is 400 pages in published form, that's four million pages. If the published novel is 500 pages, that's another one million pages of paper.
The larger the print run, the faster this paper count adds up. A print run of 20,000 double the number of extra pages, etc.
Publishers tend to be picky about how long a first novel can be for just this reason. The more paper used, the more the publisher stands to lose if the novel flops. Because of this, first time writers are usually restricted to writing novels from 80,000-120,000 words.
If you first novel sells reasonably well, however, you have then proven you can write things the public wants, and the publisher will then usually let you get away with writing a much longer novel because the risk of a flop is much less.
So it isn't that your second published novel must be longer, or even should be longer, it's just that if your first novel is successful, the publisher will then be much more inclined to take a second novel that is twice as long as the first.
You should aim to make any novel the best possible length for the story you want to tell. But if you have a 250,000-300,000 saga of a novel you want to write, your odds of having it published will be much greater if it's your second novel, and if your first novel did well in sales.