I happened to trad the README.md and saw this line
The maximum message size, including header, is **128 bytes** by default. This is configurable via `MQTT_MAX_PACKET_SIZE` in `PubSubClient.h`.
but I looked at one of the messages I was sending it was 269 bytes long (??) so I went and looked into PubSubClient.h and it looks to me that the default is now 1024 bytes,
// MQTT_MAX_PACKET_SIZE : Maximum packet size
#define MQTT_MAX_PACKET_SIZE 1024 // need to fix this here, because this define cannot be overruled within the Arduino sketch...
Just thought you would want to update the README.md