It's not that much more and anyone who has really delved into the specs would factor in the weight difference of the two. The only real benefit we have going on here is better off the line power, but with the weight we're working with, that comes to no surprise.
Not really sure what you are talking about since the Honda Rebel is the heavier bike.
Not to mention weight wouldn't change DYNO results.
What it shows is the Rebel will have a tiny bit more torque at the line, but the minimal difference and increased weight of the rebel will likely negate that.
It also shows a lower horsepower almost across the board and also quite a big drop in peak horse power. Peaking at a lower RPM than the CBR but not having near the top end speed.
To be honest I think there is a reason for this and sadly it isn't a good one. From what I read, the original plan was only going to include a release of the Rebel 500.
The 300 was only added really to fill the need in asian markets and such. And will be sold globally because it would make a good starter bike.
But the thing is the bike design was based on the 500cc engine not the 300. And while they tuned the 300cc in the same way they redid the 500 the results were not the same.
The Rebel 500 has more Horsepower than the CBR until all the way at the top RPM where the CBR has a higher peak horse power.
And the Torque on the Rebel 500 is higher until it gets into the higher rpm range making it much better in city.
So with the Rebel 500 you get a bike that would actually outproduce the CBR500 in the City and actually out produce it on the highway except for top speed.
But with the Rebel 300 you get a bike that likely would not be able to keep up with the CBR300 outside of first gear.
Not that the 300 will be a horrible bike or anything, it will still be fine. But it just clearly seems like its design was just an after thought.