Balancing partitions on a mixed cluster

I’m attempting a mixed cluster of some large (128GB RAM) and some small (15GB RAM) machines, per advice from another thread. How will the migration algorithm balance the number of partitions to each node? Are available disk/memory percentages balanced, or something else?

data will be evenly balanced to each server. so if you have servers with 32GB of ram taken, you should be adding many servers (10 seconds apart) to ensure replication does not finish before you are done adding servers.