Mind you I have limited riding experience on a CBR 3 R but myself I found the brakes quite good on this model...better than the R 3 actually.
The brakes on the smaller bikes are simply smaller and less aggressive. Yes the stock brakes are pretty good but they aren't anything close to say something like a GSX-R 600 for instance. Much better master cylinder on top of most parts in the system, braided steel lines, better/larger calipers etc. You would likely find a lot of brake fade with the ones on the CBR300 if you ride it "hard." Seems to be what other user was talking referring to.