That's just how it is. When converting a outfit and having it conform to a body, Bodyslide applies the tris (what allows the mesh to deform with a slider) that are already present on the body. The outfit will then deform in the same way the body does; if the breasts are "scrunched" together (ie like cleavage), then the outfit mesh will be too. There are two ways around this (in my experience):
1) when conforming the outfit to the reference body, deselect the sliders you feel like would cause what you don't want to happen, though the results may not be what you want.
2) Delete the reference body and save it as just the outfit, that way you can adjust the outfit without the reference body being affected, This way is more difficult because you have to edit the esp file to make the clothes not use the Biped 33 slot. It's not hard at all once you get the hang of it, but if you're just starting out it can be overwhelming.