i need help with mesh editing.

After you added more pics, I checked the file in-game and its a different issue. Something happens upon export that most of the hair becomes assigned to the breast bone, so it changes the size with the breasts.
- Select s4studio_mesh_2, with - button, delete all vertex groups but b__Head__ and b__CAS_R_Breast__;
- double-click on b__CAS_R_Breast__ and paste b__Spine2__ instead;
- Tools - Weights - Normalize all, with Lock Active disabled



Also, there a problem of flipped normals, which happens when you mirror mesh. If you were to disable Backface culling, you'd see that most strands are blue-gray (inverted). Disable limit selection to visible, press B to select a portion of them, Ctrl L to select all of them, then Mesh - Normals - Flip;