Having acknowledged the surprisingly different way the game was played with the different rackets, I followed up with the directors about it, and they explained that it was certainly part of the challenge in getting their stars properly prepared and playing authentically.
While modern tennis is played with aerodynamically designed rackets with large head sizes, the equipment that was used back in the early 1970s was far less sophisticated.
As Jonathan Dayton noted, however, while Steve Carell had plenty of tennis experience before starring in Battle of the Sexes, he basically had to relearn the game for the movie.