Compare the surface area-to-volume ratios of the Earth and Venus.

If you consider that the equatorial bulge is minimal to affect our calculations the following is true (I use ' ' o< ' to mean proportional to)
area o< r²
and
volume o< r³ .
Hence the "ratio of area to volume" o< 1/r .
If your premise that the rate of cooling is proportional to "ratio of area to volume" then it is also
o< 1/r .
What you got [i.e. 0.949] is the ratio of radii of Venus to that of Earth. It is damned obvious that it should be so.
Next, your premise is drawn from the (Newton's) 'Laws of Cooling'. But that was when these planets were red hot cinders when they started their life as 'Planets', not any more. Now the atmospheric absorption (& composition) plays its role to keep the surface temperature what it is.