That is correct...the wider firewalls were used to attach the vac formed cowls. However, it wasn't because the fuselages were slimmed down. The models simply were designed this way. Examples are the Lucky Racer (a 50s design, before Scientific started counting pennies), Cessna 172, Flightmaster, Air Champ, Gee Bee, and a lot more. Other kits had smooth, non overhanging firewalls, like the ME-109, Stuntmaster, Little Mercury, etc.
At one Musciano Meet, I talked to the son of Mr Frisoli, founder and owner of Scientific. The way Scientific saved $ was fiddling with the wings, not the fuses. As time moved on, the newer models had 1/8" sections, not the 3/16+" ones the 50s designs had. Also, the newer models usually had 18" spans, not the larger ones often used in the 50s, such as the Bullet, Cyclone, the larger F-51, Piper Cub, Stinson Voyager, etc. And, some models had their spans reduced over time. Examples include the Thunderbird, Little Bipe, F-51 again.
The margins were razor thin for Scientific. Fractions of a cent saved on materials added up, considering the large volume that Scientiic enjoyed then. But all good things come to an end (ask O & R, which seemed to have an unassailable position as engine makers in the 40s...full page ads each month on the back covers, etc).
Currell