My puppy's are fighting in the crate today!?

i put my puppy's who are a blue heeler/mini Aussie in the crate last night for bed and they slept and i took them out this morning to use the potty and i let them play in the room until they got tired and then i put them in a small cage and they went to bed but when i went in their to get something i walked out and then the bigger one was biting the little one so he got mad and started biting back the little one is the more aggressor and likes to beat up his brother what do i do?