I have four ecommerce websites (and a few other sites too). The busiest site gets more than 3,000 unique page views per day - yet it earns only a little bit. My smallest site gets around 500 page views per day and yet it's my biggest earner.
I can't figure out this "choose-a-niche-then-monetize-it" thing.
It definitely looks like your smaller site has better content, and it is probably better marketed to its target audience. In this case, less seems to be more.
YOu haven't mentioned what keywords or niches each of your sites is within. I've noticed that some keywords might be really busy - but they pay low. Other keywords are quieter but they pay so little it's barely worth aiming at them.