Why photoshop files are growing so much?

72ppi is irrelevant, once it is known that 600px X 1500px is the base image size. Presuming RGB mode with no layers or alphas or other fluff, the new doc size would be 2.57MB in Photoshop (obviously saving the file will often have a different size due to file compression).

Check to see if the vector files have guides in them and try removing the guides, I seem to recall an issue with guides in vector smart objects but I'm not sure if that was resolved or not.

It could also be an issue with excessive photoshop:documentAncestors metadata bloating the file:

https://prepression.blogspot.com/2017/06/metadata-bloat-photoshopdocumentancestors.html

File size is not and has never been a consideration in Photoshop.

This is professional software, optimized for speed and efficiency. It is not optimized for file size or users with limited storage. It is your responsibility to provide Photoshop with the necessary working environment.

You mentioned neither whether the SO are vector or pixel content, what the pixel-dimensions are, what the bit depth is, … so ultimately your description is close to useless apart from the fact that Smart Objects matter when it comes to size (as expected).

Please provide the file in question. 

Hi make sure your units are pixels it depends on page size also...regards