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Nope, there is no direct relationship between the size of the eye lens and eye relief.
There are excellent eyepieces available in both 1.25 and 2 inch formats, just as there are poor eyepieces available in both formats.
As Rob stated, the reason for having the larger 2" barrel is to allow the bigger light cone necessary with low power wide field eyepieces.
In a few cases, there are 2" eyepieces with focal lengths not requiring to be 2" format for field size reasons, but rather mechanical reasons due to having large glass elements in the design, eg Televue ethos and the old Meade series 4000 UWA.
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