How to reproduce:
1. Open the attached Unity project named "Tilemap size".
2. In "Tile Palette" window press "Paint with active brush" button. Paint some tiles in the scene.
3. Press Play. Check tilemap size.x and size.y values in "Console" window.
4. Exit Play mode.
5. Erase edge tiles or all tiles.
6. Press Play. Check tilemap size.x and size.y values in "Console" window.

Expected result: Tilemap size is lesser after erasing the tiles from the edge. Tilemap size is 0 when erasing all tiles.
Actual result: Tilemap size does not change when the edge tiles or all tiles are erased.

Reproducible with - 2017.2.0a4, 2017.2.2p1, 2017.3.1p4, 2018.1.0b10, 2018.2.0a4

Note:
In the 2018.2.0a3 version of Unity, the tile is not seen in "Tile Palette" window, but you can still draw it.
Tilemap was first introduced in 2017.2.0a4, so no older versions could be tested.

Resolved by design:
The size of the Tilemap represents the largest bounds which was previously covered by the Tilemap. For performance reasons, erasing tiles does not automatically reduce the size of the Tilemap.

To ensure that the size of the Tilemap represents the actual bounds of the Tilemap, the user should run Tilemap.CompressBounds() through scripts or from the Tilemap component settings menu "Compress Tilemap Bounds".