This is what Weiss 2020 writes on cognomina in -a (in III. *eh2-stem suffixes):
Aemilius Alba, from albus 'white'
Calua, from caluus 'bald'
C. Mucius Scaeuola, from scaeua 'left-handed person'
Weiss also mentions Klingenschmitt 1992 - I'm going to re-read this and add more later - who explains these cognomina as collectives with the meaning 'family of X' ...