Is there such a thing as too much work for a Master's Thesis?
It is not clear how to judge the "size" of a thesis (page length or word count is probably not very good), but one can clearly do too much research for a Masters or Doctorate. That said a factor of two difference is not really that big since there is always going to be a healthy overlap between the "largest" Master thesis and the "smallest" PhD thesis. If your Masters thesis is larger than the average PhD thesis and larger than the vast majority of Masters theses, then it is possibly too large.

As a student you need to be asking yourself why you are getting the qualification. Most students put in much more work than that which is required to simply graduate. You should make sure you push your supervisor to help you achieve your goals.