The library you're using (Adafruit_MQTT) has a buffer size (holding the complete MQTT packet) of 150 bytes. So your payload must be considerably smaller than this (at least using this library).
Every MQTT library I'm aware of has an upper limit of the packet size (PubSubClient: 256). Of course you can change that packet size limit in the library but I would think about your concept. Using MQTT to transfer larger XML strings is probably the wrong technology. The clever thing in MQTT is that some client publishes some values and another client subscribes exactly the values it's interested in. For XML content I prefer HTTP as the transfer method.