Is this accurate?
I was reading reviews of the Game Boy Micro a few years ago (written when the Game Boy Micro was released in the early 2000s). Many described the product as "as small as a cell phone". I found this funny—it's significantly smaller than smartphones today—including the original iPhone, by the way, which wasn't small when it came out.
I'd say the trend has been towards larger phones as we've begun to use them more and more. If you use something most of your day, you don't mind so much if takes up more pocket space.
You may be right in terms of thickness, although I think it's too early to call that a trend.
Blackberries were never that small, and weren't trending downwards in a significant way.