Do you think the similarity is that both STEEM and HIVE are aggressively traded on one exchange mostly?

I compared them, because they are based on the same tech. Isn't Hive a recent fork? It means that both have a similar product line which means they appeal to the same kind of customers. The alignment of their prices trajectory is logical as you can get more or less the same on both chains as the services are mostly interchangeable.

My conclusion would be that their correlation will become weaker the more they grow apart, or the more customers go for one of the two. I'm not too much into this, but right now it still looks to me like it's undecided, whether one or the other will win this.

When you say there is more capital in STEEM, how much more?

According to Coinmarketcap, Steem has a cap of 85 million USD and Hive has 214 million USD. So, they're both relatively similar in that regard, although Steem is much more traded with close to 15k USD compared to not even 3k USD for Hive.