There has been a long-term trend towards larger watches, with a recent spike. If you go back to the 1940's Rolex bubblebacks that were maybe 32-34mm size, you'll see that even over the very long term the average watch size has gone up. With Rolex and Patek now coming out with watches over 40mm, clearly the established brands think this is more than just a passing fad.
I think the perception of what defines a 'normal' sized watch has changed. I think people look for a larger ratio between watch size and wrist size now than they did 10 years ago.
Particularly in the US, people like anything that is 'big', supersize meals, SUVs, etc. I think it's the same mindset.