Mac storage basics · User accounts, voices and system extras
How do I remove downloaded voices for Spoken Content and VoiceOver?
Short answer: In System Settings → Accessibility → Spoken Content, open Manage Voices for the System Voice and untick voices you don't use. They can be downloaded again any time.
Higher-quality voices are downloaded on demand and some are large. Open System Settings → Accessibility → Spoken Content, click the info button next to System Voice (or choose Manage Voices… from its menu), and untick voices you don't use to remove them.
You can download any voice again later from the same place. Voices used by VoiceOver are managed in VoiceOver's own settings in the same way.