I won't get into the tightness debate. I will say that using smaller drivers offers several advantages though, mostly related to having a narrower baffle. A narrow baffle of course being more advantageous for imaging/soundstaging, as well as pure looks. I also suspect cost may well play a factor, as has the emergence of the dedicated subwoofer.
In any event...yes a 15" driver with 7.5mm xmax kills a 6.5" driver in 50Hz output. But then again, those are primarily used for professional applifications, and by those who want to go deaf by the time they reach 30. A fair comparison? Nah.
However, for the sake of amusement, lets take a look at another professional grade bass guitar speaker. Still overkill for home usage by most reasonable people, but ohh well. Driver in question is the Eminence Basslite S2012 Neo 12" Bass Guitar Speaker. No I didn't go searching for the weakest bass guitar driver in the world either, for the naysayers. I simply hit up parts express and looked at their one and only 12" bass guitar driver. I'd suspect its still a bit overkill for home usage, but ohh well.
In any event: to keep in line with the above numbers, surface area is 113 square inches. Dividing that by 33, you get 3.4, which is still quite an advantage in cone area. Xmax of the speaker is 5.2mm. So effectively, you'd need a 6.5" driver with an Xmax of 17.7mm to keep up. I don't have any of those available, but the Adire Extremis 6.8 drivers in my speakers have a 13mm Xmax, which narrows the gap considerably. Two of them in a box would certainly make the 15" big boy worry a little bit.
Then add a subwoofer. Say you cross over at 100Hz. The typical XO in your receiver will high pass with your speakers at 12dB/octave roll off beyond the -3dB downpoint of 100Hz. This means output will be 15dB down by the time you hit the 50Hz mark, drastically reducing what your mains have to do there.
Now as far as my subwoofer goes, it has a 10" driver with a roughly 25mm Xmax. It should be pretty effective at 50Hz, just going by those calculations. And of course, my subwoofer is just the entry level model....