I think that's why I prefer certain contours before others. The "grip" on the lip needs to be similar to be comfortable, but I also feel the maximum and minimum rim sizes I need, have to be practiced a lot to work. Not nessecary all sizes in between but the maximum size and the minimum size, and then personally I need to put work in just ONE in between. That work makes it possible to play all the others too as long as the "grip" on the lip is familiar.
I think I have more than 100 mouthpieces and I switch regularly to find what best fits certain horns. I can play anything from Bach 11C to Bach 1 1/4 G but certain mouthpieces like the 7C in between does not feel comfortable. That one has definitely another contour. The Bach 12C I loved for many years does not fit me nowdays either because of the "grip" or contour as Chris says. A Bach 12C Mnt Vernon on the other hand works great, and that's because it's not like a modern 12C. The "grip" on the lip feel different on that MntVernon.
I agree completely with Chris that same contour of the rim is what makes it easier to me too and not at all size, not where I am in development, and to me when the contour differs it does not mean the mouthpiece is unplayable, just it needs a lot more work and is "not as easy" to play.
When I played alto the 12E was my choice. It was the best mouthpiece for me and that Bach 39 Alto but unfortunately that 12E also has another "grip". I think that's why I needed at least two weeks of work on that Alto mouthpiece whenever I had a gig later when I had turned away from the 12C on tenor. It was much easier when my favourite mouthpieces on tenor and alto had the same contour which was the case when I played alto a lot in the early 90'ies.
/Tom