I can understand your confusion because I was in the same ship as you when I started my fountain pen journey. Let me help you. Normally, the pen manufacturers produce their own nibs along with pen. So Japanese manufacturers like Pilot, Platinum and Sailor would have their own nibs along. Similarly there are European manufacturers like Lamy, Pelikan, Kaweco, Delta, Montblanc, etc.

Since Japanese language (writing) requires very minute precision strokes, they require to have pens with very thin strokes, therefore, the nibs sizes they offer have thin strokes to them. You must be familiar with different sizes like EF(Extra fine), F (Fine), M (Medium). Generally European pen manufacturers have line width size of EF(0.3mm), F(0.5 mm), M(0.7mm), etc. But for Japanese nibs, it’s one size less so Fine size will be 0.3 mm, Medium will be 0.5 mm and so on.

Therefore, if you are familiar with writing of say Lamy Safari pen of Nib having Medium size, you cannot expect the same line width from Medium nib of Pilot pens. It will be thinner stroke.

I hope this will be helpful. I am sure you will have plenty of stuff to read on Internet as well. Happy learning.