1/3 cup does seem awful little, especially for a full day. I don't really measure their food. I free feed mine, there's food down all day...they never gobble it up, just nibble throughout the day but I figure they're not eating any more than 1/2 cup each all day. I feed home cooking for dinner, which they gobble up, but the kibble stays down all day, I change it everyday though. I've never had a problem with any being overweight either. So I guess you should do what's best for your little ones...I'd hate to give too little and they're still running around starving.
Do they still seem hungry with only having 1/2 a day? If not, I'd still give them that....1/3 seems to be not enough in an entire day.