https://www.theverge.com/2017/5/1/15...ffects-bilquis
in the mind of the visual effects designer, it was a growing/shrinking process.
Quoted from the article's interview:
"The team mapped the scene out in storyboards before testing actual shots to figure out what would — and wouldn’t — work. Haug says they focused on “the idea of her growing during the process, him shrinking, the way the room would sort of help us see her, but not see it too clearly.” It required a lot of experimentation. "
and again:
"“When we're up over her left shoulder looking down at him as he's being absorbed, it was really the trickiest one,” he says.
There’s quite a bit of hip wriggling on Bilquis’ part as the scenes moves forward. On top, she commands a position of power during sex. She seems to grow in size before the viewer’s eyes. But her hip sways, which help her devour her prey, factor into how the scene works as well. "