Hmm. On reflection, it appears that Paul E. is right. Re-organizing the types as Akim suggested does increase the size of a stack item slightly (4 bytes or 10% on a typical 32-bit architecture). This is because the embedded union is 32-bit aligned, leaving 3 bytes of padding after yyisState. Come to think of it, I suspect this is why I organized things this way in the first place.
OK, thanks to both of you.
