How can you tell which flights feature Economy Plus seating?

I got situated in the tiniest seat imaginable from CHI to Newark. But as I made the flight back, I noticed that everything was much roomier. Is there any reason why they can't offer the same size seats on every 7 series aircraft? It shouldn't be luck of the draw every time you ride with them.

How can I tell which seating arrangements are going to be featured on my flight?
 
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