Treating this as a serious question - a breast is a heterogenous collection of tissues with different mechanical properties. There’s fat, glandular tissue, skin, and connective tissue. The connective tissue portion of the breast, the Cooper’s ligaments, are sort of like the membranes in citrus fruit. They are not discrete cables or bands like the ligaments that hold joints together, but rather are more like a web or netting that runs throughout the breast from the clavicle and pectoralis muscle fascia to the dermis of the overlying skin.
Each of these tissues has different mechanical properties. The ligamentous material does not display a linear tension response to stretching like an ideal spring, and even if it did you would have to factor in the weblike network of these ligaments and the fact that they encapsulate tissue that is itself nonlinearly squishy. So no simple modeling. A breast will not deform smoothly to an infinite extent - it will stretch only so far before the forces required become extreme and the tissue tears instead of stretching (see mammogram clamp)
A lot of the extreme bounce that you get from Upton strutting on the runway is not coming from her breasts per se but from the breast tissue compressing against and rebounding from the less elastic bra or bikini top. Unrestrained, they would stretch more and rebound less dramatically, and would display a lot more lossy modes, like swinging side to side (see arm slap). It helps that while strutting her breasts experience brief, high-magnitude forces spaced far enough apart in time that the pressure waves have time to travel across the breast mass once or twice before the next big impulse. If she were shuffling her feet you wouldn’t get the same effect; ditto if she were taking very slow or quick steps. No doubt there is a step frequency that maximizes bounce by matching the breast’s harmonic frequency - I don’t know if that is a reasonably achievable pace, but I bet it would be uncomfortable. I suspect that Upton knows how to walk in such a way as to accentuate or diminish the effect, just like lots of other women.
In free fall Upton does not experience the major impulse that she does when her heel strikes the floor while catwalk walking. She is not slamming against the walls of the plane (I wouldn’t want to either), and she can’t twist her back quickly enough to match that kind of acceleration. So her breast tissue doesn’t experience the kind of forces that would either cause the breast tissue to reach its elastic limit and rebound violently, and/or her garments in this shoot behave differently, being inherently more elastic or looser. So you don’t get the ‘bouncy bouncy’ effect, but rather a gentle wobble.