The small diameter symbol shown with a Diameter dimension is a product of using a TrueType font for the Radial dimension text (Arial Unicode MS for example). All of the TrueType fonts actually display this way.
The diameter symbol looks small when using Arial Unicode MS for example as the Symbol Font File. The Symbol Font File defines the font that is used to display all text control character symbols such as the diameter symbol in this case. Changing the Symbol Font File to NX ANSI Symbols will show a larger diameter symbol.
Note - The Symbol Font File only applies when using Standard Fonts.
See NX Help--> CAD--> Drafting--> Drafting user interface and preferences--> Fonts and control characters--> Standard fonts for more info.
To increase the symbol size go to Menu --> Preferences --> Drafting...
Common --> Lettering --> Symbol --> Symbol Font File = NX Ansi Symbols
Save the change and restart NX to see it.
Note - The customer default for this (Drafting Standard --> Annotation --> Symbols --> Symbol Font File) is a Scope = Part setting meaning that it will be inherited from the template parts for new files.
Notes and References
OS Version: 1064