The decision to have some Powermates 1.25” and others 2” is a Tele Vue call. They could have easily all been 2” or all 1.25”. If the 5x Powermate was in a 2” barrel it would have had a larger clear aperture that it does now so it could accommodate the larger field stop diameter possible in a 2” eyepiece. Similarity a 4x Powermate in a 1.25” barrel would not need the clear aperture it currently has since it only needs to accommodate a maximum field stop in an eyepiece of 27mm (max for 1.25” barrel) and not 46mm (max for 2” barrel).
See
https://www.televue.com/engine/TV3b_page.asp?id=214 for field stop diameters of Tele Vue eyepieces. The eyepieces with the same field stop diameter give the same true field of view on the same telescope. Magnification will vary with eyepiece focal length.
The field stop diameter is simply the linear true field at the focal plane. The wider it is the more sky you see (for a given telescope). To get the true field in degrees you multiply the field stop diameter by 57.3 and divide by the focal length of the telescope.