No worries and thanks for replying so I could re read what I mis wrote.

Higher tuning requires more port area, not lower tuning. That was a mistake on my part, so sorry bout that.
I'm not an acoustical engineer so I don't really feel comfortable trying to explain the details of why but, I'll try and hopefully someone more knowledgeable will pop in and correct my if I'm wrong or, affirm.
With high tuning, more air is moved in a given slice of time so more port area is required in order that port noise is minimized. Likewise, higher xmax means more air is moved, regardless of frequency.