Yeah, what he said....
Seriously, among vintage American-type bikes, the heavyweights /balloonbikes are 2.125"+ tires and are usually cruisers, the middleweights are cruiser-type bikes which pretty much always came with 1.75" tires from the factory (many middleweight frames can fit balloon tires, though...) and lightweights are any bikes that came with tires <1.5" wide.... this pretty much goes across all tire sizes (20, 24, 26, 27"...) and lightweights were (rarely) cruisers, but more often commuting bikes or road bikes... In a nutshell, it all comes down to the width of the tires.
In more general cycling circles, "lightweight" can mean a number of different things, depending on who you ask, the criteria may well be different.
Personally, i think that a lot of vintage "lightweights" have tons of character, even if a Schwinn Racer or Collegiate is ANYTHING but
lightweight. MY favorite Schwinn i've ever owned is my Super Sport, and my "Holy grail" Schwinn is still probably a Paramount track bike.... any yea, really, so log as it's Chicago-built. Lightweights are cool, but by far the least popular amongst RRB folks, overall. Middleweights are popular, but a lot of effort is put into finding ways to squeeze balloon rubber between the stays.... I've seen a lot of cool "Custom" styled bikes, on this site and elsewhere, sporting at least one skinny tire....