[I have a close family member that works in publishing]

As far as I can tell, part of it is that the bigger publishing houses are able to get lucky more often; they're more likely to get a bestseller than a smaller publishing house. This is apparently due to a combination of

1. Being able to pay more authors as a raw number (they're numerically more likely to get a "hit"), which is partly because of

2. Inertia from already having big authors, so people look to sign with them before considering the smaller publishers.

Note that this is really only for mass-market stuff; specialty genres are different.

The big publishers are also able to cut better deals with distribution centers and booksellers.