I think we should be careful with `collectionBatchSize` in context of email collections, since they're pretty different.

IIRC in activity aggregation, it applies to the number of routed activities to collect. Then from there on the number of activity streams is either equal or smaller.

In email, it's going to potentially be the number of users whose full week of notifications are being gathered into memory at once. 1000 may be OK with our load right now but that number would likely need to move independently of the aggregate bucket collection size. I wouldn't mind just hardcoding this to something like 10-50 at a time for now.