It seems like you have two objects "Cube" and "Cube.001". I assume that the "Cube" has dimensions 2m x 2m x 2m.
The "Cube.001" object consists of two default cubes of size 2m x 2m x 2m but because they are in the same object the object dimensions are calculated from their bounding box as can be seen in attached image. You can separate the cubes in edit mode (select "Cube.001" and press TAB button) by pressing "p" and selecting "By Loose Parts".
If you go to File > Defaults > Load Factory Settings, this should reset everything including your cube. Granted if you have any custom layouts, you'll lose those, but at least your cubes will be 2m³ again :-)
Although, try this first: open a new scene and create some cubes. If they are still messed up then try the above solution. Otherwise, you likely hit some keys which which triggered the bottom left property panel / adjusted your default cube settings, and can be ignored moving forward.