When I import .stls in the app to edit, they somehow are increasing in size (dimension))

When I work on an stl and export it, I want the stl to stay the original dimensions but it is much larger than whenI began working on it. After Iexport it, I have to import it into another program, meshmixer, to resize it. Any help on keeping it from resizing? Wonder if it resizes it when I zoom in on the stl to work on it??