Why will some images pinch down to however small I want, and some won’t?

Why will some images pinch down to however small I want, and some won’t? I use my Samsung Galaxy Tab-S for referral to images when painting on rocks. When I begin, I have to freehand draw the image on the rock, so I measure the available area to get proportions correct. Some images I can “pinch down” to the exact height (ie. 2-1/2″) and they stay there, but some will spring back to at least halfway when I turn them loose. Please tell me how to solve this. I’ve tried reducing the resolution via Photoshop, but that doesn’t solve it. What is the common denominator to this problem, please?