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Pixel density is the same because the resolution is lower on the 38mm model, just as the resolution is lower on the iPhone 6 versus the 6 Plus.
 
This is what I keep finding when searching:

38mm 1.32" screen 272×340 pixels at 326 ppi
42mm 1.5" screen 312×390 pixels at 326 ppi

Pixel density is the same because the resolution is lower on the 38mm model, just as the resolution is lower on the iPhone 6 versus the 6 Plus.

True, though the iPhone 6 Plus has a much higher resolution than the iPhone 6, enough so that it also has a higher pixel density at 401ppi (vs 326 for the iPhone 6)
 
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