That is one half of the explanation. The other half is how projected area scales with an objects size verses how mass scales. For a given dimension d, projected area scales with d^2, while mass scales with d^3.

For example, consider a sphere:
projected area = pi*r^2
mass = ρ*(4/3)*pi*r^3

So, the ratio m/A (which from your equation above is proportional to Vt) is greater for a larger object than for a smaller one, given constant proportions and composition.



+1