Sietse,  Thanks for your detailed explanation. With your help, I have known the reason. Also, I am reading the new chapter on fonts.
 Best regards,Tim> From: sbbrouwer@gmail.com
> Date: Wed, 17 Apr 2013 10:11:19 +0200
> To: ntg-context@ntg.nl
> Subject: Re: [NTG-context] Mismatched font size when switching fonts in the chapter title
> 
> Hi Tim,
> 
> [PalatinoRoman sa 1]
> This means "PalatinoRoman scaled at 1 x bodyfontsize. The document's
> body font size is 12 pt, but chapter headers usually are ... like,
> about 2.4 * body font size? Hence the size mismatch.
> 
> [PalatinoRoman sa *]
> This means "AnotherFont is PalatinoRoman scaled to 1 x the font size
> at time of calling.
> 
> Compare these two MWE's (I've changed the font name to
> texgyrepagellaregular so that the example will work for any ConTeXt
> user.)
> 
> \setupbodyfont[12pt]
> \definefontsynonym[PalatinoRoman][texgyrepagellaregular][features=default]
> \definefont[AnotherFontSaOne][PalatinoRoman sa 1]
> \definefont[AnotherFontSaStar][PalatinoRoman sa *]
> 
> \starttext
> \chapter{Fonts and \AnotherFontSaOne Fonts}
> Generalized \AnotherFontSaOne Generals. (Constant size)
> 
> \chapter{Fonts and \AnotherFontSaStar Fonts}
> Generalized \AnotherFontSaStar Generals. (Adapt to current size)
> \stoptext
> 
> I got this information from page 5 of the Fonts chapter of the
> long-coming manual. (Not the same as the document detailing what you
> can do with OTF and Lua, although I think that one, too, is called a
> fonts manual.)
> http://context.aanhet.net/svn/contextman/context-reference/en/co-fonts.pdf
> 
> Cheers,
> Sietse
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive  : http://foundry.supelec.fr/projects/contextrev/
> wiki     : http://contextgarden.net
> ___________________________________________________________________________________