  A non-local-thermodynamic-equilibrium (NLTE) level population model of the
first and second ionisation stages of iron, nickel and cobalt is used to fit a
sample of XShooter optical + near-infrared (NIR) spectra of Type Ia supernovae
(SNe Ia). From the ratio of the NIR lines to the optical lines limits can be
placed on the temperature and density of the emission region. We find a similar
evolution of these parameters across our sample. Using the evolution of the Fe
II 12$\,$570$\,\mathring{A}\,$to 7$\,$155$\,\mathring{A}\,$line as a prior in
fits of spectra covering only the optical wavelengths we show that the
7200$\,\mathring{A}\,$feature is fully explained by [Fe II] and [Ni II] alone.
This approach allows us to determine the abundance of Ni II$\,$/$\,$Fe II for a
large sample of 130 optical spectra of 58 SNe Ia with uncertainties small
enough to distinguish between Chandrasekhar mass (M$_{\text{Ch}}$) and
sub-Chandrasekhar mass (sub-M$_{\text{Ch}}$) explosion models. We conclude that
the majority (85$\%$) of normal SNe Ia have a Ni/Fe abundance that is in
agreement with predictions of sub-M$_{\text{Ch}}$ explosion simulations of
$\sim Z_\odot$ progenitors. Only a small fraction (11$\%$) of objects in the
sample have a Ni/Fe abundance in agreement with M$_{\text{Ch}}$ explosion
models.
