Formula cars had round steering wheels back in the 60, 70, and I think even in the early 80s. They have to be small because the cockpit is narrow and also because it is an advantage for a more direct and quick steering. I suppose the designers realized that the drivers never had to remove their hands from the 9-3 position, also because the turning ratio of that kind of cars was much smaller than other type of cars, so designing more squared wheels offered advantages, such as for instance allowing a better view of the instrumentation (not so important nowadays because everything is included in the steering wheel, even the screen). I suppose that making a more flat steering wheel and not so rounded is better so that it doesn't interfere with the driver's thighs. It also saves some weight (take a look at the minimalistic rims used in DTM). In my opinion, for anything outside the modern F1 and GT3, non-rounded steering wheels are useless, and particularly absurd (and even dangerous) for street cars. For fun driving, the more rounded the steering wheel the better.