I asked a question which got very little attention due to its specific topic, so I offered a 100 rep bounty for it. After the bounty being set, this one guy was spending a lot of time helping me, and the problem turned out to be way harder than I had originally thought. I'd love to give him another +50 or even +100 rep, but apparently I have to double the bounty amount from last time. If I offer too little (100 instead of 150), why am I'm forced to give too much (300)? This ended up with me not giving another bounty when he deserved one.
I suggest adding a way to directly reward a bounty to a person without having to double the reward's size. To prevent abusing of the bounty system by giving additional attention to the question, this bounty should go "silently" to the other person without any bounty notifications.