White papers traditionally can be almost any length. I've written some that are 3 pages long, others that are 15-20 pages. Each with variation in technical information, ranging from "Here's what's important to know in this knowledge domain" to documentation of a proprietary programming language.

While some of these are written by analyst firms, quite a few are not. They're work-for-hire, often by techies or journalists who know the area in great depth, and who can ask the in-house experts the right questions and turn those answers into something genuinely useful. (Well, when they're good. Mine are good. :-) )