  The distribution of the sunspot group size (area) and its dependence on the
level of solar activity is studied. It is shown that the fraction of small
groups is not constant but decreases with the level of solar activity so that
high solar activity is largely defined by big groups. We study the possible
influence of solar activity on the ability of a realistic observer to see and
report the daily number of sunspot groups. It is shown that the relation
between the number of sunspot groups as seen by different observers with
different observational acuity thresholds is strongly non-linear and cannot be
approximated by the traditionally used linear scaling ($k-$factors). The
observational acuity threshold [$A_{\rm th}$] is considered to quantify the
quality of each observer, instead of the traditional relative $k-$factor. A
nonlinear $c-$factor based on $A_{\rm th}$ is proposed, which can be used to
correct each observer to the reference conditions. The method is tested on a
pair of principal solar observers, Wolf and Wolfer, and it is shown that the
traditional linear correction, with the constant $k-$factor of 1.66 to scale
Wolf to Wolfer, leads to an overestimate of solar activity around solar maxima.
