I think I have an answer.
the Royal Microscopal Society (RMS)
https://www.sizes.com/tools/microscope.htm
Quote:
Eyepieces
The RMS established a standard for the inner diameter of the tubes into which eyepieces fit in 1882, revised in 1889.
The standard eyepiece has an outer diameter of 23 mm.
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It's handy that you can mix and match different eyepieces ( and objectives) from
different microscope manufacturers.
Still - if there was some advantage in making larger eyepieces I'm sure that they would have done it.
My guess is that wider eyepiece design would only have the detrimental effect
of increasing optical aberrations seen in the eyepieces.
cheers
Allan