How to reproduce:
1. Open the attached "1040109_repro.zip" project
2. In Project window, locate two sprite files "Sprite_1" and "Sprite_2" (in Assets/Sprites folder)
3. Make sure that they have different Pixels Per Unit
4. In Tile Palette window, create new tilemap palette
5. Drag a few elements from "Sprite_1" into the new tilemap palette
6. Drag a few elements from "Sprite_2" into the palette, so that they surround previously added elements
7. Select all elements from the Tile Palette window and add them to the Tilemap in the scene

Expected result: Tiles in the Tilemap of the Tile Palette window should be well-sized like the ones in the scene.
Actual result: In the Tile Palette window, some of the tiles are of different sizes than others and fail to fit in the grid.

Reproducible with - 2017.2.2p1, 2017.4.4f1, 2018.1.2f1, 2018.2.0b5

Notes:
- Placing already generated Tile Assets in the Tilemap Palette results in the expected behavior.
- After placing the tiles on the Scene, the issue occurs in the Tilemap of the Scene as well, if one of the sprite files gets its' Pixels Per Unit modified.
- It is not certain that the issue is reproducible on 2017.2.3f1 due to another issue (case 1010103).
- Could not be tested on 2017.1 (because Tilemap was introduced in 2017.2).

Fixed (by design):
The Tile Palette with default settings has Automatic Grid sizing set. This attempts to resize the Grid such that the first Tile will fill up the Grid Cell if possible. This results in the fact that the other Tile does not fit in a Grid set up for the first Tile.
Instead, Tile Palette can be set up with Manual Grid sizing. This can be done during the creation of a Tile Palette or selecting the Tile Palette asset, picking the Palette Settings sub-asset and selecting Manual from Cell Sizing.