Replicator C++ exception: "properties excessively large"

From a forum post:

I am using the replicator in Couchbase Lite 2.5.1 on iOS with Objective C.

It all seems to work fine for the most part but recently it has started failing and it gives the following error in the Xcode log

CouchbaseLite Replicator ERROR: {DBWorker#3}==> litecore::repl::DBWorker ->wss://{sync-gateway-address}
CouchbaseLite Replicator ERROR: {DBWorker#3} Threw C++ exception: properties excessively large

BLIP will throw this exception when a message is created or received if the size of its properties (headers) would be > 100KB.

The question is, what would cause the properties to be that large? I'll look at the protocol docs and see what might trigger this.