I believe its due to an increase in users, the number of questions/answers, etc.

As musing continues to grow more people are finding their way here. This is turn is leading to more questions being asked and many more answers being given.

As musing is currently manually curated that means there is a lot more work and effort required by the musing team to find those gems worthy of upvotes. However they have the same pool of funds to disperse.

Human nature is such that generally you want to hold something back, to have a reserve fund for emergencies, so where before the musing team might have only scanned say 100 answers at a time, they were more likely to offer larger payouts knowing there was a limited amount of answers to go through.

Now however, they might have to scan say 1000 questions which would lead to more cautious reward behaviour as they would want to ensure they have not used the "pot" completely by question 100 thus leaving nothing (voting power wise for example) for the remaining 900 questions.

As more people join and ask/answer questions that same power has to be spread thinner amongst all the great questions/answers. This leads to smaller votes being given per vote simply to ensure the "pot" can go further.