The code is too big for Arduino pro mini atmega168

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I got an ESP8266-01 as master and 2 arduino uno, 1 arduino nano, 1 arduino pro mini atmega328 as slaves and everything is perfect. I love it.

The issue comes up with 2 arduinos pro mini with atmega168 the normal code is too big for those arduinos, the lack of memory make them impossible to work fine. Someone knows how to get the code more smaller please, I would love to put them to work together.

Thanks in advance.