  Aims. In this paper, we aim to study the time dependence of sunspot group
areas in a large sample composed of various databases spanning over 130 years,
used state-of-the-art statistical methods.
  Methods. For a carefully selected but unbiased sample, we use Bayesian
modelling to fit the temporal evolution of the combined umbral and penumbral
area of spot groups with a skew-normal function to determine the existence of
any asymmetry in spot growth or decay. Our primary selection criteria
guaranteed that only spot groups with a well-defined maximum area were taken
into account. We also analysed the covariance of the resulting model parameters
and their correlations with the physical parameters of the sunspots and the
ongoing solar cycle.
  Results. Our results show that the temporal evolution of well-observed
sunspot groups that reach at least 50 millionths of a solar hemisphere (MSH) at
their maximum can be fitted surprisingly well with our model. Furthermore, we
show significant asymmetry - described by a skew parameter of fitted curves -
between the growing and decaying phases of analysed sunspot groups. In
addition, we found a weak correlation between the values of skew parameters and
the maximum area of sunspot groups and their hemispherical latitude.
