Firstly as others noted you can use a transport-layer level compression mechanism like gzip. The other bit sort of comes down to application level logic. You

Firstly as others noted you can use a transport-layer level compression mechanism like gzip. The other bit sort of comes down to application level logic. You can filter down to the unique items and store them in a json array (or object) and in the other places instead of repeating the item use a value to reference the items (like index in the array or a key in the object).