I have used two spinning reels of almost same capacity this season with different arbor size.
One is the Daiwa Tierra 3000 with a 55mm arbor and another a Mitchell 300 Xe with a 45mm arbor.
As I was using identical braid lines on both reels, casting was really easy on both reels and so can't tell much difference.
However, line pickup after casting was more clean with the larger arbor Daiwa.
With the smaller arbor Mitchell, I was having more problems of line getting laid on the face of the spool when not careful during initial retrieve.
Sometime this line would loop off and get tangled with the line out. Kind of hard to describe, but it was a very annoying problem with the Mitchell.
My guess is that the smaller size of the arbor made line pick up less clean.
Also another thing to consider is that with smaller arbor size, there is less space for the drag disk washers.
For same capcity reel, larger arbor reels will have a more robust drag system because everything is bigger.
Daiwa's washers have almost twice the area of Mitchell washers and are more powerful/smooth as a result.
That 10mm difference, which might seem small, translates into a much bigger drag disk.
Something to consider when targeting powerful fish like salmon and steelhead.