Reduplicating an adjective +
的 (AABB or AA-
的 in this case) softens and intensifies the description, like English 'nice and round' or 'all rounded.' Compare
圆 ('round'),
圆圆的 ('all nice and round'). Same pattern:
红红的 (rosy),
大大的 (great big),
慢慢的 (nice and slow).