= Solution
Count atoms before comparing cell <volumes>. The molecular conventional cell has eight atoms; the metallic face-centered cubic cell has four. Thus
$$
v_{\rm mol}=\frac{0.726(0.480)(0.978)}8
=0.04260168\,\mathrm{nm^3},\qquad
v_{\rm met}=\frac{0.51^3}{4}=0.03316275\,\mathrm{nm^3}.
$$
Relative to the molecular phase,
$$
\boxed{100\frac{v_{\rm met}-v_{\rm mol}}{v_{\rm mol}}=-22.16\%.}
$$
The transformation therefore produces \b[about 22.2% contraction per atom] on the stated extrapolated-volume comparison. Conversely, the molecular <volume> is about $28.5\%$ larger than the metallic <volume>; these percentages have different denominators. Compressibility would be needed to turn the extrapolated comparison into exact <volumes> at the transformation <pressure>.
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