Excessive File size of photoshop files with almost no effect and edits

»I opened an image and duplicated it twice and converted them to smart objects to perform some edits. I saved the file and the file size was 787MB!«

First off: The number 787MB in itself should not be shocking when editing images professionally. 

And as you have made no mention of the image’s pixel dimensions and bit depth the number is not particularly meaningful yet. 

But as concerns Smart Objects and size: 

Assuming you save psd with maximized compatibility (and ignoring compression) a flat image is basically just »itself«.

But even changing the background layer to a plain pixel layer will roughly double its filesize because it now contains the original image as a Layer and a »preview« of it. 

This also applies with Smart Objects. 

So converting that Layer to a Smart Object results in a file roughly four times the original filesize (two. Layers, two previews). 

Whereas the flattening the Layer in the Smart Object itself will leadd to a file roughly three times the original filesize. 

»I opened 1MB image«

What were the image’s file format and pixel dimensions?