That's a pretty normal thing. If you don't use a ROM which supports Delta Updates and/or don't have it enabled, the OTA Updater always loads the complete ROM Zip. Normal thing.Ok, then I don't get why we don't need to download/flash the whole rom. I haven't used AOSP Rom for a while so I guess things have changed. That's great. Keep up the good work, I used your rom on the lg g2 5 years ago and I feel like the rom is more mature and stable now, even though we had more tweeks/options back then.
Also to your other Post: Treble Support is only possible if the Device Supports it - Treble depends on the OEM, at least if you don't use unofficial Ports, which normal don't get any Support since misbehavior on the Device could be anything but a Problem of the ROM itself.