@Senshougahara ether they can change their forms in whatever fashion they want (as they have dragon form too) or what was mentioned as a joke is true (that their father cast the spell on them what forced them to come back to him) as this flashback wasn't the only one where they were shown as grown ups (like rose dragon driving a car to splatter water on their father, mentioning that she was an actress, showing 7(stupid) brother changing from dragon form to walk an island where we saw his foots and they were unmistakingly the size of adult person).