If I capture an image online using Apple’s screen capture app (CMD+Shift+4), the resulting .png file when dragged into a DT formatted note will appear much larger than its capture size. The Apple app’s setting which cannot be modified, captures at 144dpi. If I drag this .png into an Apple email, it appears as expected. It’s only when I bring the .png into Photoshop to change the dpi to 72 that the image appears at the “right” size in DT. So I guess the question is why is display size affected by the dpi setting of the .png or any image? Usually this would only matter for print.
HTML-based formatted notes don’t use the DPI information currently. One workaround might be to enable the scaling of image attachments (see Preferences > Editing).