Due to currently existing bug - shrinking claim is possible only if at least 3 new tiles are included in the "temp lands":
This means that claim can be downsized either by starving it out/removing it and then claiming required size or by covering unclaimed tiles with those 3 steps:
Imagine we need to downsize the claim in the example to green 7x5 zone.
1st step - We are resizing claim to 8x4 zone.
2nd step - Waiting till next in-game day for changes to be applied.
3rd step - Resizing claim to required 7x5 zone.
We are working on fixing this bug - Thank you for your patience an understanding.