Canon announced the RF 70-200 f/2.8L in February 2019. This is notable for being much smaller than the equivalent EF lens, being perhaps two thirds of the length, yet maintaining the speed of the EF version - see for example the fourth picture on this page.

My perhaps naive understanding was that while the short focal plane distance of mirrorless cameras allowed for smaller retrofocal lenses, the advantage was mostly lost for telephoto lenses. Have I misunderstood this, or have Canon just optimised the RF lens for size, thus presumably giving up something else - if so, what?

Lens-rumors.com claims that the US-patent #20190004296 is for the RF 70-200mm F2.8L IS USM. This patent states:

Focal length [mm] | 72.00 | 135.00 | 194.99
Length of the lens [mm] | 172.73 | 227.50 | 242.86

^{Found in the patent application's p. 7}

So the RF-lens is probably extending with increasing focal lengths.

The official length for the EF 70-200mm F2.8L IS USM III is 199.0 mm and it is not extending.

However:

^{Stolen from ephotozine's hands-on article}

That's more than 2.6 cm - I'd say that it is closer to 5 cm (~ 2 inches), so something in the numbers is off.

I further browsed through the illusive patent application and I found three different tables for lens sizes:

[Numerical data 1] Zoom ratio 2.71
-------------------------------------------
Focal Length | 72.00 | 135.00 | 195.00
F-Number | 2.87 | 2.91 | 2.96
Tot. Lens Length | 172.73 | 227.71 | 242.23

[Numerical data 2] Zoom ratio 2.71
-------------------------------------------
Focal Length | 72.00 | 135.00 | 194.99
F-Number | 2.92 | 2.92 | 2.92
Tot. Lens Length | 172.73 | 227.50 | 242.86

[Numerical data 3] Zoom ratio 2.71
-------------------------------------------
Focal Length | 72.00 | 135.00 | 194.98
F-Number | 2.90 | 2.90 | 2.90
Tot. Lens Length | 172.73 | 218.46 | 231.71

[Numerical data 4] Zoom ratio 2.71
-------------------------------------------
Focal Length | 72.00 | 135.00 | 194.98
F-Number | 2.89 | 3.15 | 3.35
Tot. Lens Length | 207.73 | 207.72 | 207.71

[Numerical data 5] Zoom ratio 1.95
-------------------------------------------
Focal Length | 100.00 | 150.00 | 195.00
F-Number | 3.93 | 4.02 | 3.96
Tot. Lens Length | 167.73 | 207.60 | 231.68

This totally beats me...

It is possible to image using a modest single element lens. Sorry to report, the resulting images will be second-rate. That’s because all lenses suffer from aberrations that degrade. Opticians mitigate aberrations by combining numerous lens elements. Some are positive (convex) and some are negative (concave) as to power. Additionally some are cemented together; others are air-spaced. It takes all this to mitigate aberrations. Nevertheless, residual aberrations always remain.

If the camera were to be fitted with a single element lens and focused on a distant vista, we could take a measurement from the center of the lens to the image plane. This value is the focal length. In a complex lens array, finding the point to make this measurement is more obscure. The point we need to find is called the rear nodal.

Opticians can and do shift the position of the rear nodal. Now a long lens is one that has a long focal length. The longer the focal length, the more magnification it will deliver. A long lens is very desirable if you are into sports or wildlife or the like. However, you might find a long lens to be somewhat awkward.

Opticians have a trick up their sleeve that physically shortens the lens barrel. This is accomplished by shifting the rear nodal forward. If the optician desires, a complex array of lens elements can be constructed so that the rear nodal falls in the air, forward of the front element.

Remember, the focal length is a measure taken from the rear nodal to the image plane. The advantage of such a design is a shorter, less awkward barrel length. Let me add, a true telephoto design differs from the long lens in that the telephoto is foreshortened as to barrel length.

Also, you should know that short wide-angle lenses often place the rear lens group too close to the image plane. If true, there is no room for the mirror mechanism of the SLR. The optician, desiring more room for the back-focus distance, will shift the rear nodal rearward.