Look at the velocity of the pellets, in one’s chosen load. The longer shotshells are not, necessarily, throwing the individual pellets any faster, if I remember the figures. OK, so, if the pellets are moving at about the same speed, regardless of shell length, well, then we can look at the total number of pellets, contained within the magazine, with four 3” shot shells, versus five 2.75” shot shells. For numbers folks, this might help answer the question.

Personally, I have, normally, loaded 2.75” shells into my mag tubes, at least with pump guns, regardless of mag tube capacity. (With my Benellis, there have been times that I administratively loaded 3” shells directly into the chamber, such as when there were occasional, persistent shortages of my preferred 2.75” shells, compared to the 3” shells, so I did not want my comparatively-precious 2.75” shells’ rims to get chewed up by repeated feeding and ejecting. This was when I worked big-city police patrol, night shift, when chambering rounds was not an unusual occurrence.)