That may be a factor, but it is not the only factor. When applying for a spouse visa, someone from (say) Norway has little economic incentive to gain the visa. As a general rule of thumb, their visa will be subject to less checks and the marriage assumed to be genuine. They just don't have much motive to lie or try and fake their way through.

Someone moving from (say) Cambodia, however, will face an entirely different situation. They plainly have any number of motives to get the hell out of Cambodia and into a big rich country. These motives can and do lead to fake marriages, fake information, everything. The application will be given a far greater level of scrutiny and will take a lot longer.

Hell, don't take my word for it, Australian immigration spells it out. Here's the official "assessment level" (ie, assumed risk) of people applying for student visas here:

http://www.immi.gov.au/students/students/chooser/574-nonjs.h...

Plan on a miserable experience if you're not from a country in level 1. And you can be sure there's a much more detailed one for internal use, for spouses, etc.