I am using \amsbook with package \mathptmx. The fontsize of all Greek letters in math mode is about 12% larger than for non-Greek. What is it that controls the default size of each font, please?
You can't, I'm afraid. Not without substantial work, at least.
and if we look at
omlztmcm.fd we find
Now we can look at the output of
vftovp zptmcm7m, we see
(VTITLE ) (FAMILY UNSPECIFIED) (FACE F MRR) (CODINGSCHEME TEX MATH ITALIC) (DESIGNSIZE R 10.0) (COMMENT DESIGNSIZE IS IN POINTS) (COMMENT OTHER SIZES ARE MULTIPLES OF DESIGNSIZE) (CHECKSUM O 21074766077) (SEVENBITSAFEFLAG TRUE) (FONTDIMEN (SLANT R 0.277) (SPACE R 0.0) (STRETCH R 0.0) (SHRINK R 0.0) (XHEIGHT R 0.440991) (QUAD R 1.0) (EXTRASPACE R 0.0) ) (MAPFONT D 0 (FONTNAME psyro) (FONTCHECKSUM O 37333711466) (FONTAT R 1.0) (FONTDSIZE R 10.0) ) (MAPFONT D 1 (FONTNAME cmmi10) (FONTCHECKSUM O 1350061076) (FONTAT R 1.0) (FONTDSIZE R 10.0) ) (MAPFONT D 2 (FONTNAME ptmri8r) (FONTCHECKSUM O 32105367571) (FONTAT R 1.0) (FONTDSIZE R 10.0) )
at the start and later
(CHARACTER O 13 (CHARWD R 0.630994) (CHARHT R 0.4935) (CHARDP R 0.016992) (CHARIC R 0.082996) (MAP (SETCHAR C a) ) )
which means that
\alpha points to the
a glyph in
psyro (an oblique version of the standard PostScript font Symbol).
You have to rebuild the virtual font files for
zptmcm7m by doing something like
(MAPFONT D 0 (FONTNAME psyro) (FONTCHECKSUM O 37333711466) (FONTAT R 0.89) (FONTDSIZE R 10.0) )
but also all metrics should be changed accordingly by hand, so the code for
\alpha should become
(CHARACTER O 13 (CHARWD R 0.56158) (CHARHT R 0.439215) (CHARDP R 0.015123) (CHARIC R 0.073867) (MAP (SETCHAR C a) ) )
and so on for all other Greek letters.
mathptmx package, with the accompanying fonts is essentially a kludge for being able to emulate Times fonts with the limited resources available twenty years ago. Nowadays you can get better results with