I saw a fellow fountain pen user ask a question about the diameter sizes between my Opus 88 Mini and my Opus 88 Omar. I thought I would show a few pictures and try my best to show the differences!
When you read the specs about the diameter in mm, it might appear to be a slight difference between two fountain pens. When you put each pen in hand, I can say that there will be a noticeable difference between these two pens.
It’s better for me show in pictures and then try to describe what you might see and feel. So here we go.
Here are the two pens side by side with their caps off.

Let’s take a look at each pen individually. Here is my pen and it appears to be a short and chunky fountain pen. It really is!

In this close up picture, the section has a slight taper towards the nib and feed.

Here is my Opus 88 Omar.

The section is also tapered towards the nib and feed and there is a slight lip at the edge.

Here is a side view of both pens. The section on my Mini is quite a bit shorter than my Omar. At the top section of my Mini there is a thick metal band.

My Mini takes a #5 JoWo nib while my Omar takes a #6 JoWo nib.

Personally, the section of my Omar feels comfortable in my hand and I prefer #6 nibs for extended writing. Every time I start writing with my Mini it takes me awhile to get used to the shorter section and the thick metal band.
I basically use my Mini for taking quick notes and mostly for sketching. I prefer to use my Omar for longer writing sessions.
I hope my picture helps some of you who are undecided whether to get an Opus 88 Mini. Would I purchase another Mini? Probably not. One is enough for me.
Pens: Opus 88 Mini Love in Bloom with Franklin-Christoph Stub nib & filled with Diamine Subzero (Red Inkvent). Opus 88 Omar with Stylosuite EF Xwing Harpoon flex nib & filled with Colorverse Hayabusa (glistening).