Yes I also think same longer bounties gives less coins but there are some other factors as well. Basically it depends on no of participants in my opinion the longer the bounty run the more participants will participate in the bounty thus the distribution will be less because of higher participants and if the project didn't meet their soft cap or hard cap then also there would be chance we will get less coins as we expect. Even your higher forum rank or friends/followers also decide the no of coins you will earn.