The reason successive bounties on the same question must be higher is that some users kept offering bounties to a question counting on the reputation they gained from up-votes to their answer. At the end, those users gained reputation because the reputation got from votes was higher than the reputation they offered for the bounty.
The same can be said for the user who asks the question, although the number up-votes given to a question needs to be twice the number of up-votes given for an answer; if I offered a bounty of 50 point for my question, I need 10 up-votes to get back my 50 points.
See "Abusing" bounties for unlimited profit for understanding exactly why the limit was implemented. Notice also the following text in the accepted answer:
Posting a bounty is a perfectly reasonably thing, a good thing (with badges too!). However, when applied repeatedly to a question it does start to represent abuse.