the hotkey you have associated with each region is the key... no matter which region you are in. there is no concept of a 'main' region. it could take longer for the graphics engine within wow to scale up and down.. but it does happen (the faster the machine, the quicker this occurs)

so.. if you are on an alt's tiny screen.. and hit the pip hotkey for your 'main'... they will swap, leaving the alt in the bigger 'main' area and the old main in the smaller region

check this thread for some discussion on how they move around:
[Keyclone] BOOBS JOB Probs