That's not `calc.exe`. I think windows doesn't ship with calc.exe anymore. The fair comparison would be the last version of mIRC that was current when the last version of calc.exe was shipped. I don't believe that mIRC would use less RAM than that... Certainly not when you were actually USING it! (including reconfiguring the size of your scrollback, for example..)
A full-featured IRC client for Windows that includes an entire scripting language and yet it consumes less RAM than calc.exe.