smart objects won't paste at 100% scale

hooliganb  wrote I've been having a problem with this for a long time. I occasionally search for an answer, but I haven't ever been able to find one. In photoshop, when I paste a smart object from Illustrator (or when I place one from a file created in Illustrator) it never pastes at 100% scale. It's always sized to some other size (right now, it's consistently going to 103.58%).

Paste and Place are different as you write.  So lest address place first.

Place by default add a smart object layer and the content of the object is the file being place in.  In the cast of Illustrator files the smart object may be a vector object.   Documents and files have a canvas size and resolution  I would think in some information stored in the image file even in the case of Illustrator vector files like .ai and .svg.  I actually don't know for sure for I do not use Illustrator.    Photoshop does not support Illustrator vector files like .svg and .ai file well.  If you open them in Photoshop they will one as a single raster layer in Photoshop.  During open you can canvas size you want the raster layer will be rendered for using vector graphics and be size properly.    So how Photoshop renders pixels for the placed image layer need to start with some size.  There is no open dialog when a file is placed into a document.  So I would think the size and resolution recorded in the .ai or .svg file must be used to render the pixels used for the smart object layers pixels.   The object pixels will be scaled to fit the canvas by default if there is a need to.  So the Document the layer is being added to Canvas Sized and resolution is the target size.  The source size is Illustrators files default size and resolution.  Its easy to calculate what the The place associated transform scale will be if the Document and the object file have the same resolution.  The percentage will be 100% or less.    If you are seeing percents above 100% I would think there is a resolution difference between the document and file being placed in.  I have never been able to figure out how Adobe scales when resolution do not match.   I also think if you resize a placed illustrator file object Photoshop will render an new set of pixel as if you did specify a size during place and vector graphic will be user during the process like it would be done opening an Illustrator file in Photoshop.  That is why Placed illustrator files are vector smart objects.

Paste is from the system clipboard.  What is in the clipboard depends on what created its contents. If Photoshop did it could be many things.  A layer a section on a layer a merge selection  or layers.  There are different type of layers and size depend on layer size  bounds. In Photoshop you may use other applications and system features to create clipboard content.   I would expect Photoshop to create new layer when you past in the clipboard and there is image data in the clipboard and the layer would be created for the clipboard content.   If what is in the clipboard is text and not image data you would need to paste that in a tool like Photoshop text tool.  However a copied Photoshop text layers will in image document data contain the text, fonts, sizes and location in the texts layers bounds.  You can copy text into the clipboard from within text edit node.   I do not believe Photoshop do any scaling when you user paste.  To paste a smart object layer you first need copy a smart object layer to the clipboard and when you do the layers associated transform is included,  If I paste it back into the same document  it will look  the same as the original and have an independent object  just like if I had used menu layer>smart objects>New Smart Object via Cop. If I paste it into a different smaller document the has the same resolution the layers associated transform shows the same scale % but the layer is larger then canvas and does not fit.  If I paste it into a smaller document with a different resolution is does not fits and the associated transforms shows a different scale percent.

These are very complex operation are asking about.  Some of the processing remains an unknown in my knowledge

There seems to be no better way. But according to your problem, I use a SLR photos to try.

If the "smart objects" material of the original size of the "long" * "wide" background material is less than the size of the "long" * "wide" total area of 100% will be displayed.

After import intelligent object, the default image is directly into the "free transform" model.

You can click on the above lock aspect ratio, and then in both numerical box enter 100, the image will be displayed in 100%.