Perhaps I remember the mustang as larger than it was because my next car was significantly smaller, a 240z, but it is at the large end of the compact cars, which have been getting larger themselves as of late, as tends to happen. Actually, most the popular cars have gotten much larger in recent decades. In 2002, the Camry was much closer in size to the Mustang, still about 6" longer, but skinnier.

I'm not sure why you compared the Mustang to an Explorer though. I mentioned the thunderbird as being comparable to trucks, and those were well over 200 inches in the 60's. It's still an exaggeration to say that's as large as a truck with a crew cab in most cases, but the 209.4" length of a 1969 Thunderbird sedan is slightly longer than a Ford F150 with a regular bed size at 209.1". They were all, by most measures, much larger than their comparable categories today, and much larger than those categories a decade or two ago.