As it happens, we’re close to publishing a document which will contain the exact information you’re asking for. In the meanwhile, though, I can provide some general numbers.
Deltas are generated automatically whenever a snap is released, so “these updates are delivered as delta updates” is true in the vast majority of cases, particularly if your device is always connected and is current - deltas are most efficient when they model an upgrade from the immediately previous version.
Core and kernel snaps see between 1 and 3 releases a month. On average deltas are about 30-40% of the size of a full download (this is highly variable but this average covers kernel, core and gadget snap delta sizes).
You can check the size of a full download for the snaps that interest you and estimate the delta size based on these numbers, and with the (admittedly quite variable) average of releases/month come up with an approximation of the data you need.
I’ll be sure to let you know when we publish the document I mentioned.