On a Kindle 1, the original ebook only allows font sizes 5 and 6 plus a font size 7 that isn't normally available, see
Extra Large Print Kindle ebooks. Under Kindle for PC, the original ebook allows selection of all font sizes.
As seagull says, a MOBI to MOBI conversion in Calibre will rescale all the fonts. This might be enough to get a smaller font size, or you may have to explicitly select a --base-font-size (e.g. 8 or 12 points) to get a reasonable size via Calibre.