People were breeding show dogs for impressive looks long before black and white photographs. A black and white photograph of a ridgeback doesn't automatically make it an example of a genuine lion hunting dog. It could very easily be a show dog and an early example of the mentioned outcrossings for impressive appearance.

Just saying, you might have more evidence these pics are of genuine lion hunting dogs, I can't see the book here and don't know. But so far nothing you've said leads me to think so. Some things lead me to think not.

People writing books about "their" breed are the absolute worst source of information about any breed in my experience. The major figures in a breed's history that worked towards getting it recognised and etc are similar. The 1920s are precisely the most suspicious time in the RR's history, that's when fancier groups started breeding them for no real reason and working towards getting them recognised as a purebreed. What this usually means, with any breed, is outcrossing is involved not for performance, but for stabilising a certain look to fit with the little story they've written up for the fake little fabrication they're working towards developing. Basically around 1920 is when the ridgeback stopped being a real ridgeback. Not due to the outcrossing (in fact, while it was the genuine article it was frequently being crossed hand over fist with whatever, cause that's how serious working dogs are bred), no but because it started being bred because it was a "rhodesian ridgeback", rather than because it was a handy mutt on a lion hunt. And so that is when it became something else. A show dog, rather than a hunting dog. 
Somewhat counter intuitively, I suppose, a dog stops being the real thing as soon as someone slaps a label on it defining it as that thing. When you try and pin a dog strain down as "breed x", that's when it stops being breed x. And for the ridgeback that was around 1920. So yeah black and white photos of big ridgebacks from around 1920 to me don't conflict with what I'm saying. In fact it's expected, that's precisely the time when people started blowing them up to look more like what they thought a lion hunting dog should look like. They were basically creating an image to sell.

Pictures of ridgebacks from around 1860 would be of more significant interest as to what the real dogs as shaped by the task were like, the dogs before they were known as rhodesian ridgebacks, the real dogs that the rhodesian ridgeback and it's legend are nothing but a mere homage to.

Have any of those pics?

I'm not saying the show dogs today are closest to the original dogs, in fact the show dogs I see here are often huge and over 100 lbs. But anyway I'm just saying the lither ones you see here and there are closer, while still undoubtedly not really accurate portrayals of the dogs they're supposed to be. You might be right that within the show dog world a second trend has come along to breed ridgebacks smaller than the ridgebacks of old and maybe added too much sighthound or whatever. I'm just saying your "ridgebacks of old" were show dogs too, they're a breed that has been a show dog for a long time compared to a lot of other breeds. Their distinct ridge on the back and the dramatic exciting occupation drew in fanciers and enthusiasts early on who wasted no time getting stuck into making a mess of things, as they do.

The reality first and foremost to remember is the original dogs were performance bred mutts and there would have been variation in size just like there is in "aussie pig dogs", however a larger bolder dog is favoured for pigs in australia than what would have been favoured for lion in africa, and that comes partly from my research on this topic and also from simply understanding what the respective roles entail and what dog type is naturally going to be favoured.