Dave:
Yes, they are different sizes. Once the railroads got past link & pins, many (most?) of the NG railroads used a “3/4 size” version of the Janney coupler. Other NG railroads used the full-sized ones.
I use the larger Kadee “G-scale” couplers on my 1:20 equipment, mostly because they are slightly more forgiving of variations in track work. Many modelers think the “#1-scale” couplers look better. Considering the 1:24 to 1:20 scale arena, the Kadee “#1-scale” couplers are very close to the 3/4 scale couplers found on many NG railroads. No right or wrong here!
Here is a link to the Kadee handout that both explains and shows the differences in sizes:
http://www.kadee.com/html/guide.pdf
The graphic at the bottom of the first page is really good.
Here is a link to some interesting information about Eli Janney, and the invention of his coupler:
http://inventors.about.com/library/inventors/bljannycoupler.htm
Happy RRing,
Jerry