This is because urad dal is used when making dosa because urad dal has less proteins compared with rice flour so if we mix more amount of urad dal then it will become less fluffy when cooked then u need batter with little amount of protein content like chana dal (gram flour) This helps in giving proper crepe like dosa which are more crispy when cooked as compared with dosa made from urad dal alone . Another reason why we prefer urad dal for making dosa is because urad dal doesn’t get soft easily even after boiling so its batter should be good as it should not break when cooked . But if urad dal is used then batter can get little loose but still dosa won’t get thick which means if u have too much batter u can cook two or three dosas at once . It depends on batter thickness what kind of batter will be good .