Nah... they made child sized bikes too. Here is a 1975 catalog photo showing models with 24" and 18" wheels:



Perhaps the larger sized bikes survived better because there were fewer tall guys to pass them along to as 2nd (and 3rd,... 4th...) owners?

The top racing models were available for import to the US in 20" 21" 22" 23" 24" 25" frame sizes (roughly 50cm. to 64cm.)... But, by the 1970s, as a nation, US males were generally a bit taller than the average European rider. So maybe more frames in larger sizes were actually brought into the US simply due to demand.

Consider too that it was customary to ride bikes with taller frames and not much seatpost showing - well into the 1980s. "A fist-full of seatpost" was the rule of thumb used by many bike shops when sizing their customers for suitable bikes. Check out the seatpost length on this bike in the same 1975 catalog...