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Typically, these results (eg. the London taxi driver study) are zero-sum: the overall weight or volume remains constant, and it's other areas that shrink.
/too lazy to read actual paper
I don't think this is actually the case.
Volume or weight don't need to be affected necessarily, it's all in the connectome. However, in the case of hippocampal atrophy there is a significant correlation between size and function, without a trade-off with other important areas. On the contrary, looking at e.g. Korsakoff's syndrome MRI scans, atrophy in one area usually predicts atrophy in other Brodmann areas.
Why would other such areas be affected negatively? Did you refer to neuroplasticity, e.g. when areas get reappropriated, as supposedly with Savants, or deaf people? That's a different - and niche - phenomenon.
OP mentioned nothing about the connectome; to quote:
Further:
Are you trying to argue about healthy people from diseased people?
Weight and volume are surrogates for the size of the connectome, just not perfect ones (synaptic pruning in adolescence, tumors etcetera).
Healthy versus diseased as a binary choice is too simplistic. While the example I used was indeed of a rather severe disease, it also applies to e.g. elderly brains versus mid-life brains, or kids versus adults.
On any major axis I can think of (age, "diseases"), observing a smaller brain area (also: a loss of neurons) if anything predicts for smaller other brain areas (also: loss of neurons elsewhere). A positive Pearson's r, not a negative one.
Why would other areas shrink? Because cranial capacity is limited? You'd think so, but I've never come across anything of the kind (other than the niche cases mentioned). If you remember this as a typical phenomenon, I'd appreciate the source.