When decreasing quota you need to be careful not to make the size too low because you will block end user's ability to save file(s) which may result in data loss.
For example, say the file share has 20 GiB of files stored on it, with users actively using it, and you reduce the Quota to 15 GiB. The users that are using the share would be blocked from creating files/writing data/appending data/etc. due to out of disk space error. This can cause crashing in some programs, data loss, unpredictable behavior, while other programs may just display an error and allow the user to choose another location to save.
I'm not aware off the top of my head of any negative effects of increasing quota, except in the case of Premium it causes cost to increase.
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