Thanks @jonwilson. I have upvoted your post and replied to it. In my opinion you are correct. "Payouts by volume" refers to the actual amount of sbd that has been awarded to all the posts that have ever been paid out for anything that was written and tagged under the respective tag or topic. In further analysis, this is another deciding factor that lets you choose if you want to use a particular tag. If there is high volume in a tag, that says to me that other Steemers are keenly interested in the topic. There's a lot of interest in both art and photography and the payouts are strong but in order to break into this genre and establish a reputation there, whatever is posted needs to be awe inspiring or something so unique that it really stands out. It can be tough to even gain a toe-hold in both those communities because there is so much high calibre work being contributed to those tags.