How archive size is calculated
Size = bitrate of all cameras × recording time. Bitrate depends on resolution, codec and frame rate: H.265 compresses roughly twice as well as H.264, and H.265+ even more on a calm image. Motion recording needs less space — how much less depends on how often something happens in the frame.
Why the real archive may differ
A busy scene (foliage, traffic, rain, night noise) raises the bitrate; a quiet corridor lowers it. That is why a 10% reserve is included, and retention is worth checking in practice after installation.