  Spectroscopic studies of Galactic O and B stars show that many stars with
masses above 8 M$_{\odot}$ are observed in the HR diagram just beyond the
Main-Sequence (MS) band predicted by stellar models computed with a moderate
overshooting. This may be an indication that the convective core sizes in stars
in the upper part of the HR diagram are larger than predicted by these models.
Combining stellar evolution models and spectroscopic parameters derived for a
large sample of Galactic O and B stars, including brand new information about
their projected rotational velocities, we reexamine the question of the
convective core size in MS massive stars. We confirm that for stars more
massive than about 8 M$_{\odot}$, the convective core size at the end of the MS
phase increases more rapidly with the mass than in models computed with a
constant step overshoot chosen to reproduce the main sequence width in the low
mass range (around 2 M$_{\odot}$). This conclusion is valid for both the cases
of non-rotating models and rotating models either with a moderate or a strong
angular momentum transport. The increase of the convective core mass with the
mass obtained from the TAMS position is, however, larger than the one deduced
from the surface velocity drop for masses above about 15 M$_{\odot}$. Although
observations available at the moment cannot decide what is the best choice
between the core sizes given by the TAMS and the velocity drop, we discuss
different methods to get out of this dilemma. At the moment, comparisons with
eclipsing binaries seem to favor the solution given by the velocity drop. While
we confirm the need for larger convective cores at higher masses, we find
tensions in-between different methods for stars more massive than 15
M$_{\odot}$. The use of single-aged stellar populations (non-interacting
binaries or stellar clusters) would be a great asset to resolve this tension.
