So far I've not seen any really tiny sites in the AdWords placement tool, although it's difficult to compare stats that show something like 10kb-100kb in ad planner or 0-10,000 impressions p/day in AdWords, being for example a search for "pets" in the "placement ideas" search box.
If however you find tiny sites, perhaps some of those fully qualified for manual placement at an earlier stage in AdSense's history. Maybe the traffic volume threshold for manual placement was lower in the past. Maybe certain smaller sites are given preferential treatment and are subject to manual approval by AdSense staff. Whatever the case, Kobayashi's experience certainly indicate a likely reason for his and my own current no-show status.
AdSense being the largest ad-placement system in existence, Google certainly earns enough revenue not having to cater for small fish:
“In Q1 2011, Google earned US$2.43 billion ($9.71 billion annualized), or 28% of total revenue, through Google AdSense.”
Maybe Google prefers not to overwhelm its AdWords customers with too many placement choices. And of course, it will almost always be the large traffic portals that generate the most income, even if some may have a lower clickthrough percentage value of any total ad-servings number.
Maybe Google finds that large media sites are less susceptible to click-fraud, thus designing their system so as to make it more difficult for smaller publishers to automatically qualify amongst manual placements.
In AdWords I prefer to pick specific sites to advertise on and I do not trust auto-placements a bit. At the same time, Google AdWords sales staff encourages AdWords customers, myself included, onto auto-placements, as this surely provides their system a greater flexibility where to distribute ads over time in order to maximize revenue, regardless where ads end up being served.