"Paul Ferguson" wrote in message

The inside diameter of the steel vent pipe is 1 7/8 inches. If you
look straight down at the vent pipe, the diameter is effectively 1 1/4
inches, because the fascia board blocks 5/8 inches of the diameter of
the pipe opening.

The house is 45 years old, and the vent has not caused any problems
that I am aware of. Any comments on what should be done?

I'd leave it alone. Home inspectors justify getting paid by "helping" the
buyer. This is fine and technically, this should not exist so the inspector
did his job. Reality is, there are some allowances in sizing to take care
of small blockages, additional drains on the line, etc. If this has been
working for 45 years, it is not worth the effort to make changes