Greater than normal force in the stressing of a word is the most common way of producing emphasis, which in American English concomitantly produces a lengthening of the stressed vowel (“That doughnut was sóoo good!”). There is, however, a slightly different way of heightening emphasis, and that is by reducing to zero the number of contiguous unstressed vowels between stressed syllables. It is, in fact, this way that has led to the supersession of the phrase “Thanks very much” over the last decade or more by “Thanks so much.” Because the combination very much is trisyllabic and tending toward an anapestic pronunciation (stronger stress on the third syllable), one currently hears much more frequently its equivalent “Thanks so much,” in which the relevant phrase is dissyllabic and prosodically iambic.
The emphasis the word so imparts to the phrase so much is enhanced by the latter’s being monosyllabic, exceeding that imparted by its anapestic alternate, with its unstressed medial syllable. In an age when all forms of linguistic hypertrophy are gaining at the expense of plainspokenness in American English, this particular case of emphasis is especially interesting because the quantitative criterion (here: fewer syllables) yields to the supervening prosodic one in exemplifying the continuing drift toward overdetermination in the contemporary language.