I updated my Xcode to the latest version and one thing I notice is that the screenshot image dimension size became small.
Before I updated it when I take screenshot using the Simulators built-in "Save Screenshot" function on
iPhone 6 Plus simulator
1920 * 1080
414 * 736
Solution 1 (from Kip answers:)
in Simulator menu uncheck the option Debug > Optimize Rendering for Window Scale.
Simulators menu chose Window->Scale->100%. (keyboard shortcut: ⌘1)
Then save screenshot (keyboard shortcut: ⌘S).