It is an interesting point. A 737/757/A320, 6-across, can seem cramped when full, whereas a 767, just one more seat across but with an extra aisle, seems much more spacious, and much nearer to a full-sized 10-across 747.
Just as an associated point, widebodies do, on average, have a greater seat pitch as well, as most widebodies are pitched for long-haul, and most narrowbodies for short-haul, which probably adds to the aura. I know there are exceptions both ways, but they are exceptions.
There haven't been many operators with standard-pitched economy seats on the 747 upper deck, but there have been a few. Wardair of Canada had their 747-200s (so smaller upper deck) laid out in this way. Although they were a charter operator and the seats pitched exactly the same way, this was definitely a cut above the main cabin down below.