I have been having this same issue, but it only occurs on assets I have uploaded to my library recently. What I have found is that any 'new' item in my library gets placed onto the map layer as a 200x200 object instead of its native size. However, when I place something I uploaded a couple of years ago it gets placed at its native size. I don't know if something was changed with how images are uploaded to the library, but my old stuff never gives me this problem. Also, items dragged onto the token, GM, and DL layer will always be placed proportionally with 150 pixels as either the width or the height (whichever is bigger in the native size). This is true for both the old and the new assets. To note all of this has been done with the grid turned off. I have tried alt-dragging items onto the map layer, and my newer assets always get resized. Since you are saying only some marketplace assets are doing this, I can only assume they were uploaded after something was changed to have everything imported to the map layer be resized to 200x200. The ones that aren't being resized were probably uploaded before the change. I have no idea if this change was intentional or publicized I am only telling you what I have found because I have been dealing with this issue for a while, and can't seem to find a solution. If someone has one I would love to hear it.