  We present a systematic evaluation of the agreement between the observed
radii of 90 well-characterized transiting extrasolar giant planets and their
corresponding model radii. Our model radii are drawn from previously published
calculations of core-less giant planets that have attained their asymptotic
radii, and which have been tabulated for a range of planet masses and
equilibrium temperatures. (We report a two-dimensional polynomial fitting
function that accurately represents the models). As expected, the model radii
provide a statistically significant improvement over a null hypothesis that the
sizes of giant planets are completely independent of mass and effective
temperature. As is well known, however, fiducial models provide an insufficient
explanation; the planetary radius anomalies are strongly correlated with
planetary equilibrium temperature. We find that the radius anomalies have a
best-fit dependence, ${\cal R}\propto T_{\rm eff}^{\alpha}$, with
$\alpha=1.4\pm0.6$. Incorporating this relation into the model radii leads to
substantially less scatter in the radius correlation. The extra temperature
dependence represents an important constraint on theoretical models for Hot
Jupiters. Using simple scaling arguments, we find support for the hypothesis of
Batygin and Stevenson (2010) that this correlation can be attributed to a
planetary heating mechanism that is mediated by magnetohydrodynamic coupling
between the planetary magnetic field and near-surface flow that is accompanied
by ohmic dissipation at adiabatic depth. Additionally, we find that the
temperature dependence is likely too strong to admit kinetic heating as the
primary source of anomalous energy generation within the majority of the
observed transiting planets.
