According to Patricia Arquette, one of the producers of the show asked her to lose some weight. Arquette refused, arguing her character had three children and (like Arquette herself) had no problem with her mom-look.
Sofia Vassilieva missed a few episodes in season 5 because she was shooting My Sister's Keeper (2009). She had to shave her head for the role and Patricia Arquette supported her by doing the same, which explains why the two of them had short hair at the beginning of season 6.
Child actresses Madison Carabello and Miranda Carabello are real-life twin sisters who both share the role of Marie Dubois. Because of restrictive child labor laws in the film industry, the sisters alternate their parts enabling them produce a greater body of work playing the same character in the series.