There is a runtime parameter called maxshowndata which controls this, to prevent excessively large items creating huge API responses.
You can change the value of this runtime parameter in a number of ways, described at the top of this page: https://www.multichain.com/developers/runtime-parameters/
Alternatively, you can get the item's data (or specified byte subranges if it is in binary format) by using a separate command gettxoutdata. The txid and vout parameters needed for this can be found in the response from liststreamkeyitems.