What are you the dimensions of the image you are transforming? I bealive transform box overlay is limited to the frame of the clip you are using. For example....
Here is the .png image with transparency.

- sshot-1213.jpg (116.14 KiB) Viewed 100 times
When I bring it into a much smaller timeline format, of 1080p, depending on the way clips settings are for missmatch resolution, in other words how will larger clip be fitted into smaller format timeline... you get proportionally scaled version to fit timeline format. But the transform overlay remains true to the frame boundaries of the clip.

- sshot-1214.jpg (231.67 KiB) Viewed 100 times
Frame overlay of the transform tool should as far as I understand, be representation of the frame of the clip you are trying to transform. It does not outline the image if its on the transparent background it outlines the image file itself, including transparency.
If you have an image of a logo for example on transparent background, but image has a very tight crop, than it might appear the transform tool is around just logo, but if you load up another image with not tight crop you will see that it follows bounds of the frame of the file itself.
If I save the same image of a lamp post with bounds of the timeline, and I export it from resolve as png, you will see that transform tool respects the bounds of the format not what is in the image, transparency or not. Always check the original clip format of the file itself,

- sshot-1215.jpg (234.8 KiB) Viewed 100 times
If you want more control than that, fusion is the place to do more advance types of transformations, cropping, rotoscoping etc.