Just about any wide angle (WA) or ultra wide angle (UWA) lens used with an interchangeable lens camera will use a retrofocus design. That does mean larger, heavier, and more complex than a non-retrofocus design. But that doesn't mean all retrofocus lenses must be equally large and heavy (and expensive).
A wide angle lens that uses a retrofocus design is essentially a telephoto lens that has been turned around backwards. Have you ever looked the "wrong way" through a pair of binoculars? Instead of making everything bigger as is the case when looking through them correctly, everything is made smaller. That's what a retrofocus wide angle lens does.
Now compare the view looking the wrong way through an 8X power binocular compared to a 20X power binocular. When looking through them the wrong way the 20X power binocular will make things smaller than the 8X power binoculars. If both pair of binoculars are based on the same basic design and use the same types of materials, the 20X pair will be larger, heavier, and more expensive than the 8X pair.
So look at it this way - a 500mm telephoto lens has to be larger than a 300mm telephoto lens if each uses the same basic design (number, type, and arrangement of lens elements, materials with the same refractive index, etc.). Since WA and UWA retrofocus lenses are "backwards" telephoto lenses, the same is true of wide angle retrofocus lenses. With the same back focus distance an 8mm lens has to be "more retrofocus" than a 12mm lens which has to be "more retrofocus" than a 16mm lens and so on in much the same way that a 200mm lens has to be "more telephoto" than a 135mm lens which has to be "more telephoto" than an 85mm lens and so on.
That's why a 35mm f/2 lens is much easier and cheaper to design and make than a 24mm f/2 lens which is easier and cheaper to produce than a 14mm f/2 lens and so on. The corollary is that a reduction in the back focus distance allows a WA or UWA lens to be "less retro focal" than another lens with the same focal length but that requires a longer back focus distance.
It is not just a question of whether or not a lens must use a retrofocus design. It is also a question of just how much reverse telephoto power is required. Shorten the back focus distance for the same sensor size and a smaller reverse telephoto lens is required for the same angle of view/magnification factor.
The back focus distance is the distance between the back optical element of the lens and the film/sensor. This measurement is independent of the flange focal distance, sometime referred to as the registration distance. As long as there is nothing in the way the rearmost lens element can protrude into the camera and be closer to the film/sensor. With SLRs the first thing anything protruding into the lightbox would have a clearance issue with is the mirror. Even if the lens doesn't touch the mirror when it is all the way down, the mirror can strike the back of the lens when the mirror swings up out of the way to allow the film/sensor to be exposed by the opening of the shutter.
The advantage the smaller mirror gives is that it allows the back of the lens to be closer to the film/sensor without having clearance issues because the part of the mirror nearest to the lens will be further back in the camera and closer to the film/sensor than a larger mirror would.