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G'day StutzJr,
The size of the barrel determines the maximum size of the AFOV achievable for a given eyepiece focal length.
The best way to describe this is an example.
Take 68deg eyepieces. The longest eyepiece focal length that can give a 68deg AFOV (apparent field of view) in 1.25" is 24mm. In the 2" format, the longest eyepiece focal length that will allow this is 42mm.
Yes, you can get longer focal lengths in 1.25", but they won't show more of the sky. The widest AFOV a 32mm can give in 1.25" is 52 deg, and it will show the exact same amount of sky, though you will see a smaller diameter image. A 40mm 1.25" will have an even smaller AFOV because of the smaller barrel.
The 2" eyepieces hence will allow you to get a wider true field of view, ie how much of the sky you can see for a given AFOV.
There are eyepieces that have a 100deg AFOV. In the 2" format, because the limiting factor is the barrel, the longest focal length here is around 21mm. Such an eyepiece will show the same amount of sky as a 42mm 68deg eyepiece, but with higher magnification.
Now, just to add more spice to this mix, there is a new range of 3" eyepiece being developed by Explore Scientific. While not new as a format, they haven't been very popular with amateurs until the recent development of 3" focusers for photography have made them a more viable proposition for amateurs.
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