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The brightness difference is not due to barrel size but focal length. Lower magnification (longer focal length) gives a brighter the image because more light is illuminating the same apparent field (image plane).
You only need the 2" format for EPs with longer focal lengths and wide apparent fields of view. The 2" allows for a larger field stop needed by longer focal length wide view eyepieces (approx FL>23mm, AFOV=70deg). Otherwise it makes no diff. It's like putting a larger diameter dew shield on your scope, or a bigger chrome pipe on the end of your exhaust pipe. Might look more impressive but does nothing for the performance.
The 1.25" format is more versatile, with cheaper accessories (filters, barlows etc). If you ever decide to get into binoviewing then you'll almost certainly want 1.25" EPs.
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