It appears the Webster may be centred on the US of it dates Humongous before Ginormous. The circles I moved in during the early to mid 60s (mostly Melbourne-based) used the latter and I recall the former arriving later. But then, I'm also an anachronism who thinks that a lot of the stuff that Australians associate with the 60s didn't actually become mainstream in Oz until 1970 so, beware.

Cheers, Rowan