"Muscle Cars, are a Midsize car, mandatory 2 dr"
HT or a Post Coupe even Sport Roof like the Torino or GT
some accepted/arbitrary nomenclature as
Super Cars or Tuned Super Cars/Tuners,
The likes of Baldwin Motion, Nickey, Bobcat, Yenko, Mr. Norms 
The smaller cars were
or compact cars even in the standards of that era, what many were called
Pony cars
like Mustang, Fairlane, Maverick, even 'cuda/Barracuda, Challenger, Camaro, Nova
or are
Sports cars
like Corvette, Cobra, Pantera, later even Vipers
they were
not defined as muscle cars
Bigger cars were
Full Size cars, yes performance oriented maybe
like the 61 409 Impala or 406 or 427 Galaxies or the Big Pontiacs etc.
but not muscle cars, not by definition
no matter how much people argue it, it just wasn't that way
I know, not out here anyway, maybe back east, but not out west
Next qualifier to be a muscle car
a Big Block & min. of 300+ hp,
a
min. of a single 4bbl (acceptable 3x2bbls or 2x4bbls)
w/dual exhaust
many where 4 speeds but not all (not mandatory)
many had bucket seat & consoles shifters on the floor too (not mandatory)
many of the best had bench seats & column shifters/autos too
most were all
affordable youth market cars
sort of stripped down less amenities/luxuries & lighter
down market models
less expensive models, with performance additions,
$2500-$4500-ish
marketed to the youth/Boomers coming of age in
(maybe 50's is questionable as definitions goes)
it's mostly recognized as 60's cars like
(Max Wedge/Ramcharger Super Stock, arguably the 1st)
61 ? to 62 to early 72-73, it was over until like 2004-5
