To celebrate the act of writing, here are photo reviews of two 'slim' pens (under 10 mm around at the grip). Notice that, with slip-on caps, 'threads' at the grip are not an issue, so the pens are equally smooth to hold. Also, though the pens may vary in size and shape, they are all about the same in girth, so the feeling of control as one grips the pen is about the same. Performance in this case it seems to me is in the nib itself.
Both pens are aging beauties but are still manufactured (the Hastil) or available (the Fashion).
The pangrams used are courtesy of Wits 'n Wisecracks: 251 Pangrams for Everyday Use by Millard Port, via Amazon Books.
Enjoy this holiday excursion into thinness.