A little more follow-up. I went onto another machine that has an older version of Acrobat Reader and I can see in the print dialog that it actually tells you the printer percentage, which helps explain what's going on. Basically, it reduces all pages to the "Printable Area" @ 93%, which explains why these Letter pages look so small. I just don't understand why Fit or Shrink can't work as it implies, meaning, that it only shrinks "oversized" pages, leaving Letter sized pages alone so they can print at 100% (or as close to that as possible).
There may be a workaround that I haven't been able to try yet, which is to use the Poster option and then check the Only Tile Oversized Pages option. That seems to leave the Letter pages alone. But the problem there is that you have to set a percentage for all the oversized Tiled pages (i.e. reduce the size) so you lose the ability to auto reduce those pages to automatically fit.
I assume that if you want to print all the pages to a specific percentage, you also have to use this Tile option. I just find it odd that these things aren't in the main dialog. It's not very intuitive to have to go there to set pricing percentages.
Are there any really good Javascript printing add-ins that anyone can recommend that deal with these kind of issues? Thanks!