If you'd be running Qalculate on a Linux desktop system, where all the "heavy" dependencies (ICU, GTK or Qt) are already present and shared between all applications, Qalculate wouldn't require 70MB.
Of course you could also provide a Win32 frontend to bring down the space requirements drastically and make it more Windows "native"; there's a well documented libqalculate for exactly those purposes.
*.exe in Everything = 11 586 objects on this machine.
At 70MB a piece that would come out to ~800GB.