  We analyse the Kormendy relations (KRs) of the two Frontier Fields clusters,
Abell S1063, at z = 0.348, and MACS J1149.5+2223, at z = 0.542, exploiting very
deep Hubble Space Telescope photometry and VLT/MUSE integral field
spectroscopy. With this novel dataset, we are able to investigate how the KR
parameters depend on the cluster galaxy sample selection and how this affects
studies of galaxy evolution based on the KR. We define and compare four
different galaxy samples according to: (a) S\'ersic indices: early-type
('ETG'), (b) visual inspection: 'ellipticals', (c) colours: 'red', (d) spectral
properties: 'passive'. The classification is performed for a complete sample of
galaxies with m$_{\textit{F814W}} \le$ 22.5 ABmag (M$_{*}$ $\gtrsim 10^{10.0}$
M$_{\odot}$). To derive robust galaxy structural parameters, we use two
methods: (1) an iterative estimate of structural parameters using images of
increasing size, in order to deal with closely separated galaxies and (2)
different background estimations, to deal with the Intracluster light
contamination. The comparison between the KRs obtained from the different
samples suggests that the sample selection could affect the estimate of the
best-fitting KR parameters. The KR built with ETGs is fully consistent with the
one obtained for ellipticals and passive. On the other hand, the KR slope built
on the red sample is only marginally consistent with those obtained with the
other samples. We also release the photometric catalogue with structural
parameters for the galaxies included in the present analysis.
