The pens I am using:
Do I speculate correctly that you are more comfortable with a larger diameter fountain pen ? How about heft ? Is a heavier pen better than a lighter pen ? Did you try different pens ? Which fountain pen have you been using ?
I love TWSBI for capacity and have inked : 2 ECO (F & 1.1 Stub), 580 (, Mini (M).
Lamy Safari with custom grind - Too light for my taste
2 Pilot VP (M, F)
To start out I was more comfortable with a large diameter pen as it was easier to control. However, larger diameter pens were also heavier for the ones I used and too much heft caused a shake in my hands that I could not control (Monteverde Regatta). This led me to the safari as it is a light pen, but it was too light and I found I was pressing the section to hard to try and "find" the pen. The TWSBI Mini was a gift to me from Goulet Pens after they heard about my condition and it was a perfect fit when posted. This why I have so many of the TWSBI.
I have small hands and find a posted ECO or 580 is to long for me. I like the VP for my condition as well because every so often I get spasms from unscrewing caps on pens and with the VP it is just a click.
Hope this helps. I am always willing to talk about my condition and what helps me.