I am not major in engineering. but not every course in mcgill is 4 credits, only 7-- and 6-- are credits, you can also select 5-- courses(3credits).
If you choose to have thesis option, u only have to have 6 courses that accumulate 21 credits and other credits are for thesis. If a project option, u have to have 9 courses that accumulate 30 credits, other credits are for projects.
I don't think there are big difference in master's requirements among engineering or science departments