A shaddah adds intensity or gemination in pronunciation. Rhetorical classification does not automatically treat every doubled pronunciation as an independent extra root letter creating a difference in letter count.
A pair distinguished by a geminated and a nongeminated sound can therefore be considered under form or vocalization rather than simple difference of number. But if actual letters or their sequence also change, the analysis must take those changes into account.
Vowel variation such as “burd/bard” and the heavy-versus-light articulation of a consonant both belong to the broader question of form, while their immediate phonetic causes differ.