I got it right because this is exactly what I am doing hourly for one of our databases, in lieu of transaction log backups (DB is 3GB in size, generates 3GB per hour of transaction log traffic. Backend for an off-the-shelf product and the vendor/creator is of little use).

What I have noticed and cannot explain, is the varying size of the differential backups. Theoretically, the differential backup only backs up extents changed since the last full backup. That would imply the differential backup file will only get bigger and bigger. However, I am noticing a more up down pattern: 18, 10, 10, 10, 18, 10, 10, 20, 10, 10, 26, 70, 72, 77, 92, 84, 85, 89, 93 and so on. Generally up (especially once people get in & use the system) but not consistently so.

Of no concern whatsoever, just curious to know why it should be. Possibly by an extent being updated more than once but a subsequent update having less data in it?