Exactly my point, Sol; many, if not most, find a limit. If you cannot see these things as Beshine does, as "enormous boobie adventures" that she loves to experience throughout her days, you reach that limit earlier. It takes a personal fascination with the size of your boobs and how they influence everything you do. We heard Beshine start talking about these things excitedly after she exceeded like 12-14k, and how she couldn't wait to expand further and experience more. Clearly, this only adds to her happiness, but not every girl may see these things as "adventures," but as problems.
If I am not mistaken, Mary wants to go well past 10k. With enormous boobs, supported high on her chest so that her slim waist can be seen, will she embrace and enjoy the changes, or will she find them tiresome? Time will tell, but my hope is that her fascination with the size of her boobs will create the situation that, like Beshine, she will love all the ways her boobs make her do things differently. Certainly, stamina is compromised, but I can imagine that when relaxing, the size and the weight create certain sensations that actually add to comfort in many ways.