It's always a bit of a tradeoff. A 512MB bucket size would double the size of the Replication Table and we want make sure we keep the entire working set in the buffer cache. 1GB also has the advantage of filling up slower, which gives us more time to close full buckets and create new ones as needed.
That said, 1GB is a bit of an arbitrary number which just happens to work pretty well. 512MB might work just fine. The exact number isn't as significant since typically we lose a whole disk at a time and thus needs to recover a certain number of TB regardless of the bucket size.
It's always a bit of a tradeoff. A 512MB bucket size would double the size of the Replication Table and we want make sure we keep the entire working set in the buffer cache. 1GB also has the advantage of filling up slower, which gives us more time to close full buckets and create new ones as needed.
That said, 1GB is a bit of an arbitrary number which just happens to work pretty well. 512MB might work just fine. The exact number isn't as significant since typically we lose a whole disk at a time and thus needs to recover a certain number of TB regardless of the bucket size.