In a racing environment, a 600 takes less strength to ride, because it's lighter. Transitioning a bike from a tight left to a tight right is harder than you would think at triple-digit speeds. Less power also lowers the chances of spinning the rear tire on corner exit and "high-siding." A high-side occurs when you get on the gas too early, or with too much lean angle, and the rear spins and then slides. The natural tendency is to react to the slide by letting off the gas. That causes the rear tire to regain traction, and the rider is thrown into the air as the bike rotates over that point of traction: the tire that just regained grip.
If you're talking about more of a sport-touring bike (Yamaha FZ1 versus an FZ6, for example), then the difference in power and weight still make the 600 "more forgiving," because you have fewer ways to make a mistake.
Where a 1000cc bike can lift the front wheel by accident pretty easily, it's not as easy to make that mistake on a 600cc bike with less power. The same is true of heavier vs. lighter. If you get a heavy bike off-balance, it's harder to hold upright than a lighter bike.
Realistically, most modern 600cc bikes are more powerful than is safe for a new rider. We are spoiled and fooled by technology in America...
Personally, I find that racing 600's is more fun than 1000's. It's easier and cheaper. (not to mention lower insurance, if you're riding on the street).
for a new rider though, look into a smaller bike. a 650cc twin is the absolute largest sportbike I'd consider for a new rider. Even that's more power than you need. Modern "beginner" bikes have as much power as racing bikes from 10 or 20 years ago. How fast are you really planning to go? a 650 twin will hit 60 in 3.5 seconds and do almost 140mph... A liter bike has DOUBLE the horsepower. Where are you going to use it?