Seems like Database font (size) is modified by ActorFontSize skinparam rather then DatabaseFontSize. Is this bug or is it on purpose?
You may try it (e.g. https://www.planttext.com/):
skinparam backgroundColor #FFFFFF-#EEEBDC
skinparam databaseBackgroundColor #FFFFFF-#DDEBDC
skinparam DatabaseFontSize 24
'skinparam ActorFontSize 24
database MyDB
vs
skinparam backgroundColor #FFFFFF-#EEEBDC
skinparam databaseBackgroundColor #FFFFFF-#DDEBDC
'skinparam DatabaseFontSize 24
skinparam ActorFontSize 24
database MyDB