I find this an irrelevant argument, because I experience no discomfort with even the smallest-diameter concertina buttons I've experienced.
At last a favourite of mine: Button size .... a wider button area does admit using stronger springs with the same (or even better) comfort. Compare the Hohner 200+ grams again and its 12mm buttons which cause me no discomfort at all.
And I don't know whose preference Göran is talking about here, since it's definitely not mine, but in any case, that's not my point in opening this new topic. I wish to say, instead, that in my own experience the following factors are more important than button diameter:
Generally speaking the 'comfort preferrence size' for 'touch buttons' use to be (10)12-15mm...
..1) Interbutton spacing: When I tried the "travelling Tedrow" concertina, I wasn't as enthusiastic as the others who were with me. Every individual factor I tested was excellent, but it didn't add up to an excellent "feel". I finally localized the "problem" as being the fact that with slightly larger than normal buttons but standard button spacing, there was less space between the buttons, and to me it's important to have that space in between. I've had a similar experience with other instruments that have larger (6-7mm) buttons, but on those it was less obvious because other factors were also different.
Some people say they have problems when playing of hitting buttons adjacent to the ones they intend. I don't experience that problem with concertinas, but it does sometimes happen to me on a computer keyboard, where the keys are more than 3 times as large. Why? I suspect it's because there is virtually no space between the computer keys, relative to their size.
..2) Location of keys relative to the palm bar and strap: I've experienced instruments (I'm speaking mainly of anglos here, but comparison with duets is also useful) where the entire keyboard is shifted either parallel or perpendicular to the bar (or both) compared to the "normal" location, and I find that only a few millimeters difference can significantly affect "playability".
..3) The curvature and "rake" of the keyboard layout. What I'm calling "rake" is the angle the "diagonal" rows in the button pattern make with the line of the bar. Again, I have experienced variations in these factors -- on both anglos and duets -- and found that small variations can significantly affect the ease of playing.
..X) Finally, all the above factors interact with each other, and varying two together may have a very different effect from either variation taken alone. This is my main pont. Discussing button diameter or any other factor in isolation may be misleading, as it can miss contextual interactions. Button diameter has been discussed "to death", yet the other factors I've listed have hardly been mentioned. I think that's a mistake.