I use Adobe Garamond Pro in combination with Termes Math for math (the
latter for missing glyphs, Garamond is not very complete). How can I scale
size of Termes Math without decreasing the size of the Garamond glyphs in
math mode?

/MJO


    % define the characters to use for Garamond in math mode
    \definefontfamilypreset
        [math:somesymbols]

[range={0x0002A-0x0002E,0x000A7,0x000B6,0x000D7,0x000F7,0x02020-0x02021,0x02212,0x02219-0x02226,0x02228-0x02265,0x02012,0x0003C-0x0003E,0x0007C},
         tf=style:regular,
         offset=lowercasenormal,
        ]

    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:digitsnormal]
    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:lowercasenormal]
    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:lowercaseitalic]
    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:uppercasenormal]
    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:uppercaseitalic]
    \definefallbackfamily[mainface][mm][Adobe Garamond
Pro][math:somesymbols]

    \definefontfamily[mainface][rm][Adobe Garamond Pro]
    \definefontfamily[mainface][mm][TeX Gyre Termes Math][scale=0.9]  % <--
scaling here...

    \setupbodyfont[mainface,11pt]

    \starttext
    100

    $100$ % <-- leads to math mode that is smaller than text, even for
Garamond glyphs
    \stoptext