  It is shown how the Equation of State (EoS) depends on nucleon properties
inside Nuclear Matter (NM). We propose to benefit from the concept of enthalpy
in order to include volume corrections to the nucleon rest energy, which are
proportional to pressure and absent in a standard Relativistic Mean Field (RMF)
with point-like nucleons. As a result, the nucleon mass can decrease inside NM,
making the model nonlinear and the EoS softer. The course of the EoS in our RMF
model agrees with a semi-empirical estimate and is close to the results
obtained from extensive DBHF calculations with a Bonn A potential, which
produce an EoS stiff enough to describe neutron star properties (mass--radius
constraint), especially the masses of PSR J1614_2230 and PSR J0348_0432, known
as the most massive ($\sim 2 M_\odot$) neutron stars. The presented model has
proper saturation properties, including a good value of compressibility.
