I would sorta agree that a liter bike is safer for an Experienced rider..i.e. the power curve..But, I disagree that any of the above mentioned bikes are good for a "new" rider, they have too much power for someone that's NOT familar with motorcycles, and how they work. New riders seem to have to many " moments" and in a panic situation that could prove to be deadly. Granted, you can get hurt, and or crash on any sized bike, heard all the arguments about that for the last 5 yrs...
The reality is, sportbikes end to be heavier than say a ex500, or a SV 650..or a simular bike, these bikes are however MUCH more User friendly, and more forgiving than any sportbike will ever be..therefore, IF the new rider gets into a bad situation, he, or she is more likely to be able to deal with it, and have a better outcome, than if the same rider was on a 600, 750, or heaven forbid a 1k..
Keep in mind, that regardless of the type of power curve these bikes have, that they get up to Speed, REAL fast, and that's a downfall for the new / newer rider, reaction reflexes ect..are to slow in that stage of their riding..Example, a new / newer rider is out ridin with their friends in the twisties, some are faster, some not..The rider is on a 750..lets say, he or she comes to a fairly tight turn, that they may have misjudged...they in their minds are going much to fast to take the turn, first reaction to grab the brake, but wait, their fingers slip off, and they accidently grab the Throttle, I've seen it happen, more than once, so now what.?
The 750 is picking up speed at a Rapid rate, the rider is in full panic mode, and depending on the surroundings, i.e. guardrails ect...that could be their last ride. Now if they were on a EX250, 500, or even a SV650, with less power, and being lighter, and easier to toss around, I believe the riders chances of surviving are way better, sure he or she may have crashed, but it prolly was at a much slower speed, and what about the panic thing, would they have had more time to react..? Would the panic been lesser..? in the same situation..? Could they have made the turn..? I think you have to agree, that in all of these cases the answer would have to be at minium most likely, or yes.
I can tell you this, I have a 33 yr. old Son, and if he were just starting to ride, there's NO way, I'd recomend anything bigger than the SV 650 @ 70 hp...and he's 6'6" and 290lbs..
So, IMHO...keeping the throttle in check idea, goes out the window...ride with discretion..same deal, you can't control your enviorment...ride with experienced riders...same deal, They can't control your surroundings, or your actions either.
There are other things to consider also, but I've rambled on long enough so that you get the idea.
One last thing...Ask 100 experienced riders what the best bike is for a new rider, and that 100 should have at least 10 yrs, and over 50k miles under their belt, I bet 98% or better say the 650 or smaller....Do you suppose there's a reason for that..?