SBD payout on the other hand accused a drop of about 27.8% probably due to less upvoting from the big accounts

That's not how the system works. If an account which used to actively upvote becomes dormant, the rewards that were formerly assigned by that account go to effectively slightly increasing the rewards of all other active upvoters. [1]

I haven't run the numbers, but I'm fairly sure the decline has been due to three effects:

• The median price of Steem over the first week of August is lower than the median price over the last week of July. The amount to be rewarded is essentially a fraction of the Steem market cap [2], so changes in market cap will change rewards proportionally.
• After the latest major release, posts now have a second payout cycle that lasts one month. I haven't run any analysis on this, but offhand I'd guess an increasing fraction of the reward fund is being eaten up by posts in their second cycle, because effectively all posts have been entering their payout cycle but none will be leaving until one month after the change took effect.
• The fraction of the market cap which goes to post rewards decreases over time until it reaches its long-term target [2].

All three forces will tend to pull rewards downward.

[1] Conversely, if an account that was formerly dormant becomes an active upvoter, the rewards for the newly active account come from effectively slightly decreasing the rewards of all other active upvoters.

[2] See the whitepaper for more.