You do understand that the maya blendshape node only stores information that changed. So actually doing the extra work of merging just something with blendshapes makes things heavier than by having not split model beforehand at all
Anyone can tell me how to delete the construction history and preserve the useful history only.
Most likely you can not do this. See the merge has to happen you cant just magically remove operations that come after the blendshape and make the blend shape stick. If you delete the merge then the face is no longer attached to the rest of the body. But as i said this is all unnecessary gimmickry.
You can undo this but now its even more work*, now you need to attach each separate face to a copy the body then blend the entire shebang**. Now the trick is that attaching does need to proceed with merge and attach in same selection order. Then apply blend to full bodies (with the delete targets option checked in blendshape options). You have now made the model lighter. Even if it feels counter intuitive, but maya only stores those vertices that actually change, see sometimes developers actually are smart,
- in essence when you decided to split of you did the mistake. This entire escapade is a futile attempt to make things easier that just ended doing things even more complicated.
** Alas i didnt think theres a easier option for extraction for the full body blendshapes and thats to duplicate all different blendhapes form your existing history then delete that and use those full bodies as blend targets.