Error when trying to update an item in DynamoDB via management console

I've got a 17KB item stored in Dynamo. If I open it in the management console (https://eu-west-1.console.aws.amazon.com/dynamodbv2/home?region=eu-west-1#edit-item?table=TABLE&itemMode=2&pk=PRIMARY_KEY&sk&route=ROUTE_ITEM_EXPLORER) and hit Save Changes button, I get the warning message below:

The update request failed with the response below. The request can proceed by overwriting the item.
Tick this box to confirm
Invalid ConditionExpression: Expression size has exceeded the maximum allowed size; expression size: 4255

I can proceed and update the item after ticking the box, but I'd like to know what is going on. What expression is this message talking about? Any help is appreciated.