Actually I had read this on a website, and observed it through a software as well that vented needs more space than sealed.
And it makes sense. For better understanding, let me explain it through example. I'm talking in terms of inner volume. For a 1 cubic feet total volume sealed box, the inner volume excluding walls/sides thickness would result in a box that is slightly less than 1 cubic feet. And when you add a port/vent to that same box, the inner volume is further reduced due to port taking up its own space. So to restore the inner volume to 1 cubic feet, you will ultimately have to increase the overall volume, which results in a greater volume.
So don't just look for the overall volume of the box, what does matter is the inner volume available to the woofer. Or in other words, a 1 cubic feet ported box has lesser inner volume than a 1 cubic feet sealed box.