From Jon:

I was looking at migrating over to the new stream but it seems EventStreams does not surface the change in bytes made by a revision.
This is quite crucial to my tool and blocks me from moving over to the new service.

Previously this was surfaced like so:
length: { new: 2, old: 1 },
where new is the new length of the page in bytes and old is the old length in bytes.

I looked too, and I see { new: 123234, old: null }, but old is always null. Not sure why, but I assume the eventbus extension somehow isn't filling this in properly, even though it uses the same code the RCStream does.