The only chubbiness on display is from looking at the picture!
She ain't chubby. I suspect the "fat" arms are padding; her ad does say they're looking for "muscle builder" type bodies for "Superboy." so they might pad "supergirl's" arms to make her seem powerful...

And, while the Supergirl movie was bad, the actress playing the title role was darn cute. The PLOT stank to high heaven and the name actress in the film overacted (persumably due to the direction given).

                    A. Leedom, BoredPresident of the RED RAVEN REVIVAL SOCIETY. Rolleyes
                         2015 is the 75th anniversary of RED RAVEN COMICS #1!
                              (Cover date August, 1940--published May 29, 1940)           
                                 Reprint Red Raven Comics #1, dangnabit!
              Marvel MUST honor the anniversary of its most mistreated hero!
                                                           Pretty please!!Cute
                                  SUB-MARINER #26,  WORSE COMIC BOOK EVER!!
                                          DESTROY ANY COPY/REPRINT YOU FIND!!