How to make clothing pieces expand with body size?

Hi there!
Sorry if the phrasing is weird- hopefully I can explain this in a way that makes sense.
Basically, the project I'm working on is a pair of pants that replaces the Sim's legs with skeleton legs. I placed a flat plane to cover the hollow center of the Sim's torso- this looks as it should in game, but only when the Sim is only at a certain body weight. If the Sim is any larger, the size of this plane does not expand, leaving a gap between it and the edges of the Sim's torso. Here's a link to a Drive file with two screenshots, the Blender file, and the .pkg file showing what I mean.
Is this something that can be corrected? Or am I kind of just going to have to cover this area while styling? I'm assuming the issue is with weighting, however I'm not sure what I would have to change in order to fix this. I'm still very new to this and way over my head lol. Also I know there's a lot of other little things going on glitch-wise, slowly but surely figuring things out!

Thanks in advance for any help you can offer!!