The smallest ship we have been on was the old QEII and the largest Iona. There have been pros and cons to each one but we have enjoyed them all.
The smaller P+O ships are older, with fewer bells and whistles, but can get into different ports and we find it easier to get to know crew and other passengers.
The bigger ships we have been on have been newer with more entertainment venues and speciality restaurants. However, the number of passengers can be a problem at busy times and the itineraries tend to be more limited.
On the whole, I would far rather be on a cruise than not!