Perspective too lossy

[Bohoho]

I'm a pure beginner. I just wanted to change perspective of part of bitmap image. I did, then I took a screenshot of the image and pasted it where I wanted it (in a different editor). I think it came out smaller dimensionally than it should have. Parts of it, that were supposed to be smooth curves or pretty straight lines were way too jagged. Any tips? Does Gimp use a lossier than necessary algorithm? Do I have to be sure to use the minimum number of perspective adjustments or are things calculated from the original image rather than the most recent lossy adjusted images?