Why GrabCAD prefer in format STEP/IGES instead of Parasolid that give small file to be upload?
3 Answers

STEP and IGES formats are universal in the international language of 3D programming.
I think all 3D programs have the option to import the file formats (IGES, and STEP) standard built-in 3D program interface. Producers in this
way to ensure compatibility with your own program with others.
PARASOLID (not the end all I know, but ) is a platform (a kind of code) on defined which is a program built on.
then the program has a plug PARASOLID

There is an "Other" category when uploading files, you could use that when uploading your Parasolid. I don't believe there is a real preference in the GrabCAD community for one file type over another. There are several questions on the topic of STEP vs IGES already, I think it's just a matter of personal preference at this point.