That's a lot of questions. but for offshore work, days from shelter, I think it's more a question of weight than size.
An ultra-light 40-footer is less safe than a 25' wooden Vertue, in my view.
Sorry but I do not agree.
As Jeff as said it is complicated. It has to do with RM but also with dynamic stability. RM has to do with weight and GZ (the arm). Bigger boats have a bigger GZ so they have an advantage and they are also bigger proportionally to the wave size, that's another advantage in what regards dynamic stability.
An older and heavier small design can partially compensate the disadvantage to a bigger modern design because the weight contributes to the RM and this one to stability, but modern designs with much lower CG (that contributes to a bigger arm as well as the bigger beam) end up to have a better overall stability.
Note that between two boats with the same stability (the same area under the RM curve), one bigger and lighter, the other smaller and heavier the lighter one will recover much more quickly from a knock down assuming they have similar RM at 90º. The force that is pulling the boat up is the same, but the force needed to put an heavy boat back in its feet is mutch more than the one needed to right a much lighter one
That is just an important factor, there are much more about it but generally we can say that a bigger boat is safer and certainly it is if it is the same type of boat. I am assuming well designed and built boats as are most of the boats built today.
Regarding the case you have pointed out I have no doubt that a Pogo 12.50 is much more seaworthy than the old Vertue by a big margin even if the displacement is not very different.