Drag and Drop vs Import Layer Image Size

I've figured it out! Ignore me. Thank you. 

Sorry, can't delete the question - incredible functionality by Adobe... 

I've done some more testing. I think I've figured out why the layer drag vs drag from desktop is different, but both say 100% in the transform tool despite being different pixel sizes on the screen: if I drag from the desktop, it is a smart object and therefore retains it's original resolution/ppi. If I open the image in Photoshop and then bring the layer across, it matches the ppi of the document I'm placing it into (which I assume involves resampling). 

My question remains: why if I place a 450x450 300ppi image into a 450x450 72ppi canvas does it come in at a much smaller size? Shouldn't it come in at a bigger size? 

Note: I am aware PPI doesn't mean anything on screens, it's only relevant to print, but it is impacting how the images I import are imported and I need to understand why, especially as these images are eventually going to print. 

Thank you!

I've just tested it with some other images. The 150ppi image comes in smallest (when dragging from desktop) and the 72ppi image comes in the largest/same size as the artboard. Given they're both the same pixel dimensions and the document they're going into is 72ppi, why should this have an impact? If anything, shouldn't the 150ppi image come in larger than the artboard???

Again though, if I open them in Photoshop first and then drag them across as a layer, they come in at the correct size (ie. if the image is 500 pixels high, it is taller than the 450 high artboard).