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.
http://www.lunduniversity.lu.se/o.o.i.s?news_item=5928&id=24890
According to Johan Mårtensson from Lund University, if you are learning new language quickly, it helps your brain to become bigger and increase its activity:
And there is more: