Why does the capacitor need to be small in RC filter circuits?
Basically, lower valued capacitors can use dielectric materials that have better performance. Less variability due to temperature and bias voltage. Less variability over time. Higher working voltage ratings.

The best materials generally only deliver capacitance values up to 100 nF or so at reasonable prices.

Here, we're talking about \$RC=1\ {\rm s}\$, so if R is 100 kohms, then C will be 10 uF. In this range, keeping the capacitor value lower (i.e. 10 uF instead of 22 or 100 uF) will allow you to keep the part physically smaller (which is good if you need to fit it on a board with a bunch of other stuff), and/or allow you to have a slightly better working voltage rating.