Sites backlinking eachother

Let's take an example from www.fitness.com
It gets most of it backlinks from bigfitness.com , how come ?

 

Relevance, search engines place more value on sites that have relevant links pointing back to them. Example, a fitness site has a bunch of auto repair sites linking back to it would not be as valuable. It maybe the difference between a pr 1 and pr 3

 

bigfitness is PR2
and fitness.com is PR1

So it does help a lot.
I'm wondering how I could use this same method myself?

 

Write articles and submit them to relevant blogs and directories that link back to your site. There are sites that specialize in this, one I know of submits them free of charge all you have to do is write the articles. It will move your site up quickly. Go to my site below articleinformation.com and look for the free traffic system text link on the right side.

 

It shows more links because its a sitewide link. It doesn't mean the links carry more weight.

 

Do backlinks lower in quality or worth if there are many from the same domain?

 

Having a backlink matters, no matter if its from a lower PR site.

 

Good deal, thanks for quick response.

 

Also bigfitness.com gives backlinks to 4 more huge fitness sites ? So basicly this one site gives out 1.2 million backlinks
Does anyone know any websites which can provide this kind of optimizing service?