Larger companies have an inherent advantage when it comes to SEO. In most cases, they have many more links coming in, more direct traffic, better marketing outside of SEO, etc. All of these things together assist in quicker success when quality on-site SEO is done. In this video, Matt Cutt’s discusses SEO for large and small websites and the difference in ranking potential. He starts by actually saying that larger sites do not have an advantage, because smaller sites are more agile. He then notes that smaller sites have less manpower than larger sites, so you should start by focusing on a niche and branch out.
Overall, he makes a good point. It is a good idea to start small and niche and build out from there. The first couple years of starting a online business are always the hardest, so it is a good idea to concentrate resources.