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Clues, anyone?
I had a site with less than 10 pages with one having PR5 with only 1 incoming link from PR7.
Otherwise the total PageRank within your site increases with the number of pages in it. This can also lead to high PR home page if you link properly.
Most links tend to go to the home page so this is where most page rank comes in. This page rank is then filtered of to pages linked to from your homepage. At this point some page rank might leave your site but most will be spread over sub pages. These subpages have a share of the initial page rank minus a small dampening amount.
At this point the process starts again with the subpages distributing their page rank. The thing about subpages is that they nearly always link back to the index page so they return a lot of their page rank back to the index page. They might link of site which means that a percentage of the page rank is lost to you forever. The amount lost to off-site links depends on how many links on the page link of site compaired to how many go on-site.
Now your sub pages might have sub pages (sub-sub-pages compaired to the index page) but these will still probably link directly to your index page and return some of the page rank originally from your index page. The net effect is that a large percentage of the page rank distributed from your index page to sub pages is returned (with dampening) to your index page.
This is where your style of navigation comes in. If you have a fixed navigation on all pages that links to all pages on your site it will lead to a fairly even distribution of page rank with any page that has external inbound links being slightly higher (due to the dampening of page rank as it's distributed and the number of iterations of the page rank calculation that have passed).
If on the other hand you have an index page which links to 'sections' which then have sub-navigation and all pages link back to the index page you will skew your sites page rank to your index page. This happens because the sub sections don't link to pages within the other subsections so the page with the most internal links is your index page.
The second style is a good way of focusing all inbound page rank onto your index page but sub pages will have a reduced page rank compaired to a site with the fixed identical navigation on all pages. This is great for targeting a particular keyword but a more even distribution can be more succesful at gathering traffic from several less competative search terms, it all depends on what you need.
The interesting things happen when google is freshly distributing this page rank as it takes several iterations of the calculation to reach a balanced distribution. During this process page rank will tend to oscillate up and down (i.e new sites page rank jumping up and down between updates).
I have also heard that google will asume a page rank of 1 for new pages it hasn't asigned a page rank to. I am not sure if this is true as I have only ever noticed distribution of external page rank around my sites but this might also be a reason for a large site having particularily high page ranked index pages.
I think the main reason that large sites tend to have high page rank index pages is because they will need to use some form of sub sections as it's not reasonable to link to 200+ pages from your index page. The more pages linking directly to the index page you have the larger the percentage of the initial page rank will be returned to your index page.
As I say I am no expert and perhaps someone will correct me on my simplistic model but this is how I comprehend what is going on.
However just adding new internal pages and linking them to the home page will have no effect on its PR, since the new pages will have zero PR. You have to get external sites to link to your internal pages before they'll make a difference.
Probably 80% of the external links to my site are to internal pages... but then I have a rather broad range of content.