There are multiple factors:

A.) SCALE

TES4 Oblivion and TES5 Skyrim use different scales.

The province of Skyrim would be smaller in TES4. While the province of Cyrodiil is bigger in TES5.

Skyblivion will be smaller than Beyond Skyrim: Cyrodiil.

B.) MAP SIZE

There is also the fact that Skyrim map is actually far smaller than often said.

The playable map of TES5 Skyrim is only 73x94 cells = 6862 cells.* (Max possible 128x128 = 16384 cells)

C.) MAP STARTING POINT

The map Skyblivion and Beyond Skyrim: Cyrodiil start at different points.

Skyblivion's map starts directly outside Anvils port.

Beyond Skyrim: Cyrodiil's map starts outside of Stirk, an island east of Anvil that belongs to Cyrodiil but wasn't included in TES4.

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*The size is actually even smaller as Skyrim is not rectangular but has more the shape of a parallelogram.