Well I don't believe boerboels are supposed to be anywhere close to the size they often are either, I share this opinion with south african farmers I've met, who think the boerboel was more like an american bulldog (performance type) than the giant monstrosity it is today.

Around 90 - 100 lbs.

This is my cousin's ridgeback x bullmastiff, it weighs about 90 lbs, if that.

Which is typical for a working cross of these breeds. Chances are both the ridgeback and the bullmastiff in it's parentage were considerably bigger than it, but it has serious work to do and so naturally adapted into a more functional size. One that probably harks back to a size it's parent breeds had back when they had to work. This tells me fairly strongly that neither bullmastiffs nor ridgebacks are supposed to be anything close to the sizes they commonly are today. A quick bit of research into the history of the bullmastiff (night dogs, gameskeeper's dogs, bandogs, etc) shows that 90 lbs was a big one, 100 lbs a huge one that would be lucky to function, but great news if it did.

Boerboels are nothing more than bullmastiffs that found their way to south africa, with undoubtedly a bit of outcrossing but to even smaller breeds so there's no reason they should be bigger. And like I said farmers tell me working ones aren't. 90 lbs or so is about right. A gripping dog is nothing if it can't support it's own weight with it's grip, and that was the boerboel's role on the farm- gripping dog.

A ridgeback as a lion baying hound I would think smaller still, you can certainly see how being nimble and quick and light on their feet was even more imperative.

I just don't see the ridgeback as being too thin or too small, the show dog/purebreed world doesn't do that, they blow dogs up as soon as function no longer matters. Happens to every breed and I think clearly the RR as well. I just don't see the benefit of being bigger and chunkier for a ridgeback. Compared to boerboels and bullmastiffs whose sole focus was on close quarter combative gripping engagements and being built to endure the punishment with them. They topped out at 90 to maybe 100 max when they were serious working dogs. Ridgeback it seems should have been a good deal lighter for their area of expertise.