This picture displays what is likely a difference in visual perception. Looking down at essentially exposed breasts to your right vs covered breasts closer to eye level on your left; Brandi's will appear more one dimensional. A graphic artist could super-impose Brandi's breasts over this of Foxy to visualize the actual difference, but I am not a graphic artist. Using a spherical calculator, inches to cubed millimeters, we could roughly find Foxy to have a radius of 5.3" while Brandi has a radius of 4.2", a diameter difference of only a couple inches.
If Foxy could have the boobs to match her dreams, lets say 20k, her radius would increase to 6.7" or 13.5" in diameter. That silly little 3" would essentially double her volume, kind of just what we see with that 2" difference between Foxy and Brandi. And using the same calculator settings, we see that Foxy was probably in the neighborhood of 6.2" when she first got her expanders, thus she has increased her diameter by about 4".
Foxy has mentioned a couple things she would like to surgically change beyond much bigger expanders and one being a lift of sorts with a internal bra. In this picture we see with Brandi, like Luna and others, that her breasts basically stem from her underarm level, giving that high and fake appearance where the crest can actually start to rival shoulder height. Foxy does not benefit from that look; there is a large, perhaps 5" gap between her shoulder and where her breast swells much lower. We can see the visual difference with Brandi's breasts high enough to show off her waistline and despite being smaller and the girls of similar height, her breasts are higher, more in your face. Functional wise, I know that Foxy would be in for quite an awakening if suddenly her breasts were raised to a similar level; not only would her posture likely make them seem lighter, but they would be above her diaphragm and lower ribcage edge, providing better breathing and comfort. But beyond that there would be the positives or negatives, depending upon which way she would view them. Higher means a higher center of gravity, so balance is more easily affected. It also means her line of sight becomes more boobs and less what is directly in front of her. And then there is arm movement; as we can see, Foxy does not experience any hinderance, while we can see that Brandi, though smaller, will have her upper arm pushed wider by her sideboob if she were to reach at all forward. They would all add up to an entirely different experience, where though easier to carry, her breasts would suddenly become far more obtrusive, requiring many more adaptions, and probably making her feel that her boobs are much bigger because their effect on her life is so magnified.
Makes me wish Foxy could experience the whole thing today; I bet she would love to be further inundated by her breast size.