the wheel base ranged from 120-165, this is I"m going to guess about 132? but it is a 4 door giving it that jump vs the 2 door J series which then also had a full size bed vs this 5ft bed. Make it a 2 door and I bet its sub 120. So by that it is about the same size but "size" isn't the only thing that defines a truck.
Width is close, at about 1" less on the new one, but a CJ to a JL has grown by about 6" in width so had a J series continued to today it would have likely grown by at least that much.
The J series had a ~2 ton payload limit depending on version vs this under 1 ton (still good but not even close to the same) So its kind of a wimp in that department and not up to the "size" of the gladiator.
The J series Gladiator was meant to compete with other full size trucks, this is not. It is still an S10 (colorado)/ranger sized truck. A modern J series would have been much larger than what this is. The name is the name clearly, nothing we say is going to change that, this just doesn't seem to really fill the shoes of the original one.