Looking at qalculate in Arch repos, libqalculate comes in at 14.94 MiB, and qalculate-qt at 3.37MiB, so 18.3 MiB total. If you're looking at the Appimage, the rest of the size would be from the libraries it depends on, like Qt/GTK+
libqalculate is the mathematics part and the command line tool, while qt is GUI, so it's surprising the GUI part isn't the larger one of the two
About 6MB of it is the libqalculate.so.22.19.0 file, 1340KiB is .xml files that define all the units/functions/etc, and another 6MB is the documentation.