I don’t have enough information to help you – what are you trying to do exactly? Export does not scale what it exports except as set by the ‘scale’ value you enter in the field there, so make sure it’s set to 100%.
The usual issue people sometimes have with Export is that when exporting a single selected object, a small amount of additional space is added around the edge, to make sure that all graphic effects have room to be rendered. This means that the shape might have a reported size of, say 30 cm x 20 cm, but the exported size is very slightly larger. This can be addressed if it’s a problem by using Slices to establish an exact pixel size for a region and exporting that. But it does not sound to me as if this is your problem.
Let me know what Export settings you are using and I will try to explain what’s going on.