Some believe that to be, yes. I personally haven't been able to prove that with my own sites so I cannot confidently say one is better than the other. But if you take two sites...
1) blog.domain.com
2) domain.com/blog
If #1 has more relevant and quality inbound links than #2, then I believe #1 is going to have the greater SE success in the long-run. My blog is on blog.domain.com and I've had no problem getting spidered. Now, would it have done better had I add a folder to my main site 2createawebsite.com/blog I guess I'll never know.
Yes, # of pages on your site is important, but it's not the only thing the engines check for. It's the total picture.