For normal ADSB on PiAware, an internet packet is sent out when it is filled up with messages OR 10 seconds has passed a partially filled packet will be sent. This minimizes the header overhead. I haven’t looked into how modeSmixer does it’s thing. If it is sending 1 ADSB message per packet I can see how this can run into data usage problems.
PiAware also has some code to limit the number of messages that can go into the packet. Like if a plane isn’t changing altitude or heading then we will block those messages from being sent.
MLAT require all data packets to be sent and the limits are removed. This effectively doubles the data requirements for MLAT site.