Following the theme of my previous post I will continue on a related topic.
I've noticed that fictional fantasies pertaining to forced feminization usually has a fairly drastic split when it comes to having a sissy with a flat chest or a sissy with huge breasts. There's rarely a time when a scenario "splits the difference" and portrays a sissy with smaller breasts or loosely fit clothing that hides the chest region. They're usually either flat as a board or DD-cup.
While people probably have some fantasy they prefer it's probably linked to their deeper fantasies about sissification. Those who wish to be women probably gravitate towards artificial breasts or implants. Those who thrive more on humiliation are likely split between the particular scenario and situation involved.
Since I tend to thrive more under the humiliation aspect than others, I will break it down from that aspect.
False breasts via breast forms, stuffed bras, and the like (or even more permanent methods such as implants or hormone therapy) tend to yield great internal humiliation but the external humiliation is a bit situational. The inner humiliation stems from them being there at all, since they wouldn't be otherwise. The added girth up top gives a sissy a great deal of self-consciousness and if performed via implant or hormones, I expect they would surrender quite deeply and feel defeated. As for external humiliation, I believe this would vary depending upon how realistically passable they are. If fully dressed, breasts would likely decrease the level of external humiliation since it makes them more passable as a woman. If partially dressed in an unconvincing manner, breasts would likely increase the amount of external humiliation as it makes their appearance more flagrant. Picturing a sissy in a very tight-fitting pink t-shirt with huge breasts but with facial stubble and male pants and shoes would stand out something harsh.
On the other side of things is the flat chest. If a sissy is almost fully passable as a woman, a flat chest will likely increase external humiliation. While the internal humiliation might lessen a bit, there would likely be some added self-consciousness if he was hoping to pass as a woman and now would worry about people noticing his lack of breasts. In regards to partial dressing (or non-convincing outfits) the flat chest would again probably have slightly less internal humiliation but also increase external humiliation. Certain artists have taken this to an extreme by portraying sissies with exposed nipples nearly all the time in order to accentuate their flat chests and male stature.
Personally, I've found a bra with no breasts to increase the level of internal humiliation and self-consciousness since at that point the bra is rather pointless and used only as a means of increasing self-consciousness.