Yes, beefing up the orchestra for cast recordings is a practice that goes back at least to the 1950s, if not earlier. Orchestras were often expanded by about 12 players, most or all on strings.
The Sunday recording is an unusual case in that the extra players were used sparingly.
Anyway, when you listen to cast recordings of Broadway musicals from the 1950s through the 1980s, you're usually hearing a bigger orchestra than was ever heard in the theatre. Even Off-Broadway musicals sometimes either added musicians or had an orchestration created just for the recording. Examples of the former include The Fantasticks and examples of the latter include Little Mary Sunshine and Dames at Sea. And it's still sometimes done but I think it's perhaps a bit less standard now than it used to be.