It can mean whatever you want it to mean to get your point accross.
The useage will vary depending on if I'm talking to a fellow enthusiast, or if I'm talking to someone like my mother-in-law.
Typically, I don't really follow the strict "big cube engine in a mid-size car ONLY" definition. We know what the classic definitions of "sports car" "pony car" and "muscle car" are, but there's overlap between them IMHO.
A "pony car" typically had sporty handling and was more Trans-Am inspired... put a Bigblock in a Camaro, HEMI in a 'cuda, or 429 in a Mustang, and suddenly they've lost much of their sportiness, and have crossed well into "muscle car" territory.
I wouldn't discount a L79 Chevelle either... they may not have big cubes, but they've got plenty of "muscle" for their day.
As for the full-size cars, anything with a solid-lifter bigblock definitely is a "muscle car" IMHO, despite not fitting the strict definition. Other lower-HP bigblocks go into a bit of a gray-area.
The definition of "classic car" shouldn't be all-inclusive to anything over a certain age... there needs to be some collectability and desire for to actually own one in order to be a "classic". An early Dodge Caravan is old enough, but it sure as hell isn't a "classic".