Bounding Box

[1] Here is the original test document, the layer pixel bounding area is smaller than the canvas. It is being placed as a smart object, before committing the placement, the transparent canvas area is indicated:

[2] However, once the place is committed, Photoshop ignores the transparent pixels and only the layer pixel bounding area is indicated:

[3] By editing the smart object and changing the original to include a white background, the original canvas size is then retained, it’s no longer cropped to transparency as there is none:

[4] Using the Layer Blend If sliders, one can remove the pure white 255 level pixels:

[5] The placed image now retains its original bounding area:

The document is untitled, unsaved, not a PNG… Unless you’re referring to the placed smart object layer as originally being a PNG?

I’m not sure if I’m understanding the question correctly.

You can transform/scale the placed layer bounding box directly… Or you can edit the smart object and resize the smart object document and then update the smart object layer.

By definition, the bounding box is the size of the non-transparent area. One thing you could try is to paint on the layer, away from the image, with a single pixel square using the Pencil tool at 1% opacity.