Indeed; if I understand it correctly encrypted data goes into a standard bloom filter. If that's indeed the case then, assuming that users can never be removed

Indeed; if I understand it correctly encrypted data goes into a standard bloom filter. If that's indeed the case then, assuming that users can never be removed, a daily update would just contain the indexes of the additional bits set by the newly registered users, which could never be 40MB. It seems they also forgot to include lowering the p-value of the bloom filter as a trade-off, since it seems that a false positive means that a contact is sent to the server which shouldn't have been. If you push that down to p=.95/.98/.99 or so you'd get a much smaller Bloom filter and many people might not object to a few % of 'collateral uploads' so to speak. Minimizing the amount of information leaked through these various tradeoffs is an interesting problem though, since you'll have to leak something one way or the other.