6.5 vs 8 vs 10 woofer

Why then are most all subs 10” and 12” or bigger?

hard to isolate

depends on implementation with many many other factors

I fundamentally disagree with the notion that there is only one approach to any aspect of speaker design.  That someone purports to have determined what diameter is ideal for a full range driver, regardless of other aspects of speaker design, such as type of enclosure and type of bass alignment (bass reflex, open baffle, sealed box, quarter wave back-loaded horn, transmission line, etc), the type of tweeter to be used with that woofer, and a whole host of other factors, speaks to the lack of experience and understanding of the OP.  That this conclusion was arrived at by watching You Tube videos, adds to the folly of this assertion.

I've heard many different full/wide range drivers used full range, or with a tweeter or with a woofer or in three way systems, and there was not one single characteristic that was common to what I considered successful implementation.  Size did not necessarily matter, type of magnet structure did not matter, cone material, you name it, did not universally determine what was best.  Yes, I heard terrific 8" full range drivers, such as the AER BD 2, used in several different systems and the sound was completely different because other aspects of design were different.  The best application I heard of this driver was in a single driver, quarter wave backloaded horn system by Charney Audio (utterly amazing); another system where that driver was used in an open baffle system with a subwoofer was not as successful to me.  The very best system I heard with a wide range driver had a 13" field coil driver in an open baffle (the 13" driver operating full range (no low pass filter cutting off its upper frequency response) with a tweeter crossed in way up high in frequency. 

There are more ways to make a speaker system than there are ways to make lasagna--don't tell me that there is only one correct recipe, even if it happens to be the one I like.