Curious if it is a rounding problem. If the model is coming out super small then the program exporting the file doesn't have a unit size or the unit size is not checked prior to export. If that happens then it may come in too large or too small for PreForm.
I am wondering if it comes in so small into PreForm it loses some of the precision. Is PreForm double precision?
So far I usually make sure my unit scale is in the file's information. If you don't have that option in your software then maybe you have it under obj export and if so try that. When exporting obj files do not weld all verticies or merge if you have multiple mesh groups and make sure you export Triangles Only.