Some people say you can see the wind flags at 50-m better within the tunnel. It also allows more light through to your eye. I guess its one of those things that what suits one person doesn't suit another.
I use 18-mm tunnel fore sight on my Walther air rifle for 10-m indoors but see no reason to change to 22-mm.
JSB
The setup of the rifle has some bearing on the problem. A free rifle with a 12 inch barrel extension a 18mm front sight can start looking kind of small. I am debating a 22mm front sight for my prone rifle for just that reason.
For my air rifle however, I personally feel 18mm is plenty.