Yes they are rather slim for work horses
Really?
As for the size of the horses, well, they varied in size and girth, but most (not all) of the WW2 war time photos I have seen show horses that are overworked and/or with malnutrition.
But is modeling warn out malnutrition horses a good thing? Depends on the gamer or modeler I guess. To appeal to a wider range I would think modeling a standard looking horse would be the good choice.
Rubicon Models horses look nice and taken care of, so this can be good or bad thing I guess. Overall though, I think it's a good choice because they could market these horses to a wider range of interested gamers including Napoleonic Wars, American Civil War, World War One, or really, any 28mm tabletop miniature gamer that would like to have some horses.