I can’t answer the question directly, but this may be related.
Back in the 90s when the cable channel TLC wasn’t shit, they had a series showing actual surgical procedures. I watched one where a woman had previously had a mastectomy, and this follow up was essentially rebuilding her breast by moving fat from her belly into her chest.
It’s not just cutting out one lump of fat and putting it somewhere else. The blood supply had to be kept intact, so it was more like a slide puzzle. Lots of stuff moved only as far as it could without interrupting the various veins and arteries.
Interestingly, they made sure to point out that the fat being shifted around still thinks it’s belly fat, so gaining and losing weight that would normally affect the belly would show up in the one reconstructed breast
Presumably this other procedure is similar. Fat is moved from the legs in such a way that by moving a relatively small amount, the butt gets bigger and the legs get smaller, making the butt look even bigger by comparison.