Arduino Uno - MQTT message using PUBSUB on ESP8266
The maximum MQTT packet size in PubSubClient is 128 bytes. After all the MQTT header information that leaves only 99 bytes for a message.

You can change it by editing the PubSubClient.h file:

#ifndef MQTT_MAX_PACKET_SIZE
#define MQTT_MAX_PACKET_SIZE 128
#endif


As it says in the PubSubClient README.md:


  
  The maximum message size, including header, is 128 bytes by default. This is configurable via MQTT_MAX_PACKET_SIZE in PubSubClient.h.