That is an easy question. It is a harder answer...
Ready for a long-ish read?
There is a LOT of confusion out there when it comes to the size of speakers. In the mobile electronics world where speakers have forever had to fit into stock car mounting holes, we can see lots of variations. The term "six-and-a-half" is more a marketing term than anything as the 6.5" coaxial exists in lots of different physical sizes. The 6" speaker is also out there as a standard designation, but often 6's are marketed as 6.5's and vice versa. If you got out a ruler and started measuring 6.5" car speakers you will find it is a random even that you find any measurable feature that actually measures 6.5 inches. The same goes for 6x9's. 4's, 5.25's etc... There is a standard out there, the EIA standard, that tries to codify the standard sizes for each general size class of speaker, but when you look at the "standard" you realize there really is no standard.
As a statement of fact, ALL speakers of a general size-class will vary in their critical dimensions, specifically grill OD, cutout diameter, bolt circle diameter, mounting depth, and mounting depth at motor diameter, (I bet you hadn't considered that one, design a truck box....).
Move on to woofers, where the 8 has been commonly only been a woofer until recently. Anyone remember Q-Logic empty enclousres? Remember the QX rings and how they promised to help you line up and center woofers that were different sizes? On the 10"-class woofer as an example the cutout hole, depending on brand and model of woofer, ranged from 8.45" to 9.25" as I recall. I vaguely remember the numbers since I was the design engineer for that patented feature of the Q-Logic enclosures. My point in bringing it up is that woofers also have a range of sizes as well. The 10" - class of woofers among all brands of woofers is the worst, with the widest range of sizes.
The 8" woofer and the 15" woofer actually have the tightest range of sizes. Put another way, most 8" woofers have about the same critical dimensions for cutout ID and screw pattern and most 15" woofer have about the same critical dimensions for cutout ID and screw pattern.
The KICKER KM8 is designed with the 8" woofer standards in mind.
Specifically, it is roughly the same size as most other 8" speakers in the market with regards to basket OD, bolt circle diameter, cutout etc.
SO..... Back to your question, what would one call a TRUE 8" speaker? From my experience the best answer is: "I don't know."
Based on historical standards I might say that the KM8 is closest to a true 8" speaker based on the long history of 8" woofers in the mobile audio market. But I already pointed out that with any class of speaker size there is a lot of variation, so who knows? The best competitive example of a different-sized 8" speaker I know of right now is going to be familiar to the Malibu owners out there. That example is the Rockford M-series "8-inch" coaxial. It has a grill OD of 9" and a 7.25" cutout. Its bolt circle is 7.75-inches. It is bigger than the KICKER KM8. The KICKER KM8 has a grill OD of 8" and a 6-5/8" cutout. Its bolt circle is just under 7-7/16"
Is the Rockford Fosgate speaker larger? yes. Is it an 8? They say so, and I don't disagree.
Is the KICKER an 8? yes. Is it smaller than the Rockford Fosgate? yes. Is it an 8? yes, certainly based on the long history we all have with woofers...
I think the big question looming in the minds of many is: "Can you take the Rockford Fosgate speaker out and install the KICKER in its place?" Right now I don't know the answer, but I bet it will be pretty difficult for many, based on the dimensions shared above.
Since the comparison has been floated and the objective comparisons have been made based on measurable data/facts, I know the next question will be: "How do the KICKER KM8s sound compared to the Rockford Fosgate 8s?" My answer will be to tell you to listen for yourselves!
Any feedback I provide based on my own experience will be subjective; it is dependent on my ears and my tastes. Anything I tell you will be MY opinion, and I don't want an opinion to be misconstrued as fact. Hey, I am just being honest!!!! We all have different tastes...
I hope I did at least an okay job of answering the question. Sorry for going on so long, but there is a history there, so it is worth understanding the decisions in context.