The video doesn't touch on two things: All those undersquare engines are turbocharged and the quirks of the V6 (it focuses on inline 3,4, and 6 and V 8, 10, and 12).
Why does turbocharging matter? Undersquare engines don't breath as well and it's because of the small bore diameter.
Put on your hot rod hat for a minute: small block Chevrolet 302 from 1969 and small Chevrolet 305 from 1991. Why does the 302 has such massive performance potential and why is the 305 such a dog despite being nearly the same displacement?
302 = 4.0 in bore x 3.0 in stroke
305 = 3.736 in bore x 3.48 in stroke
The small bore 305 shrouds the valves so much. That is, when a valve is open, it's so near the cylinder wall that it disrupts in the incoming air. It has a big metallic wall on one side of the valve, blocking air movement through the engine. That's why you never see a hot rod small block Chevrolet with a small bore: The inherent dimension of the cylinder and it's impediment to airflow limit the power potential of the engine.
I used the 302 and 305 Chevrolet small blocks as an example to keep the same engine family but it applies further. 1990s Camaro vs. Mustang. Camaro was available with 305 (5.0L) and 350 (5.7L) but Ford only had the 302 (5.0L). A Camaro with a 305 was no match for a Mustang, despite near-equal displacement. Why was Ford able to coax more power out of the same displacement? Ford's 302 also has a 4.00 in bore. Chevrolet's 350 has a 4.00 in bore. Not a coincidence on why their performance is more on par.
Bore diameter isn't an absolute and head and combustion chamber design do play a factor, i.e. all engines don't need a 4.00 in bore to make power and two small valves (as in a 4 valve engine) would have less shrouding than one large valve. The idea to take away is that bore diameter has an outsized effect on power, more than just increasing or decreasing displacement.
That is, until you add a turbocharger. Pressurize the intake and the engines (in)ability to ingest air on its own is mitigated. Turbocharging opens up more avenues for engine design because it neutralizes the airflow detriments inherent to some designs.
What about those V6 quirks?
First, understand that horsepower is a calculation of torque x RPM. If you want to increase HP you
must increase torque or
increase RPM. This is only math:
Now, on to the V6 quirks. All modern V6s use a 60 degree V for packaging purposes. It's narrow and easy to package in a tight engine compartment. Because of this angle and inherent imbalance issues with the V6 configuration, the crankshaft uses a split-pin configuration. That is, each connecting rod has it's own rod journal that's offset by a few degree from the cylinder next to it. This is not an issue for a 90 degree V6, which use a common pin for two cylinders (but still has imbalance issues and they're really wide) and it's obviously not an issue for an inline-6 engine.
To increase power, we can increase torque. We can increase torque by increasing stroke. Think of increasing stroke like having a longer lever on the crankshaft. The longer the breaker bar, the more torque we can apply to the lug nut. Same idea with a crankshaft. One problem: That wonky split-pin crankshaft design that makes a 60-degree V6 work also limits maximum stroke. Darn.
Let's look at the Toyota GR V6 as an example of stretching stroke (still not a long-stroke engine by any means):
1GR-FE - 4.0L = 94mm bore x 95 mm stroke (nearly square) - 270 HP@5,600 rpm, 278 lb ft@4,400 rpm
2GR-FE - 3.5L = 94mm bore x 83 mm stroke (slightly oversquare) - 314 HP@6,200 RPM, 260 lb ft@4,700 rpm
The longer-stroke 4.0L makes more torque but the shorter-stroke 3.5L makes more power. The key is at what RPM the power is made. Using the HP formula from above and a little math, you'll find the 4.0L makes 253 lb ft at peak power (5,600 RPM) and the 3.5 L makes 266 lb ft at peak power (6,200). Despite making a bunch more torque in the mid-4,000 RPM range, the 3.5L extends torque production further into the engine RPM range than a 4.0L.
Why not just design the 4.0L to rev like the 3.5L? You can't. The longer stroke limits maximum RPM. That big lever crankshaft weighs more and creates more force. You just can't swing it as fast.
So, to make power in a 60-degree V6 you need to rev it more. To rev it more you need a shorter stroke. To make it breath properly at higher RPM, you need a large bore to unshroud the valves.
This is how every auto manufacturer who makes a 60 degree V6 ended up with an over-square engine.
The last question is: Why? Why does all this matter?
Marketing. Every automaker needs to make a 300 HP V6 to compete with the next automaker. The 300+ HP 3.5[ish]L 60 degree V6 is like a blockbuster summer action movie: Every movie studio has to have one.