Just as an anecdotal example, I had extractions and a retruded maxilla and don't have any thinness in my lips... though my eyes are super hollow and recessed! So, maybe this has to do more with individual genetics?
You do have nice lips. Did you notice any changes in their volume after surgery?
I think genetics would influence the starting point i.e if you start off with fuller lips and have bicuspids removed you won't notice the thinning as much, there's like a better tolerance for it. But if you start off with thinner lips you will notice it more
I read that every 1mm the upper incisors go back the lip retracts 0.5mm. When there is less support the lip curls under. Every case is different though (the reasons for extractions, the amount of original crowding etc) so this is just like a blanket statement from the extraction stories online and from my own experience too.