Is it possible to reduce the maximum packet size of Xbee 802.15.4 only reducing the maximum payload size?

I am doing a research about the influence of the maximum packet size on the performance of a 802.15.4 network. Is it possible to reduce the maximum packet size of Xbee 802.15.4 only reducing the maximum payload size?

In the frames, only payload/RF data field is variable, rest of the field have fixed length.
So by reducing the payload size you can reduce the packet length.