It’s part of the crappy morph transfer system they use and it’s been like this since DS3, if your item is meant to be skin tight then it works well, but it’s pretty much useless for anything else.
You either need D-Forms or learn how to make the morphs in a modeling program.
Pretty much all the automatic converters had this problem. I’ve seen it in the Tailor, Wardrobe Wizard & Crossdresser. I haven’t used XD4 enough to know whether that has the problem too.
If you’re a poser user and can’t use the Clothing Cleavage Modifier, there are several characters by Kaposer that include a fix.
Xanna, Naika and the V4 Easy Breast System include The Fixer.