The core is a 5*5*15 ! You can't make a proportionnal 5x5x15 unless shapeways wall thickness is brought down to 0.1mm or less !
EDIT: Claus, the reason this can be proportionnal is because it is a cross 3x4x4, and two factors allow proportionnality:
1) the 3x4x4's proportionnal
2) 3x4 faces are circleable.