When calculating by volume, 40% is not such a crazy reduction in size.
Take a theoretical 250mm x 250mm x 50mm box for a volume of 3,125,000 cubic mm.
3,125,000 x .6 = 1,875,000 cubic mm (60% of the original volume for a reduction of 40%)
225 x 225 x 37 = 1,873,125 cubic mm (about 60% of the original volume)
That's a reduction of 25mm x 25mm x 13mm in external measurements which doesn't sound so far-fetched.