Hey! Thanks for reporting this - the easiest solution right now is to symlink your Mailspring preferences / data folder (where most of the file size is once you sync your mail) to another location. I’ll see if we can make this a preference soon, though we’ll probably start by adding new sync features that allow you to limit how much is stored on disk for a lighter setup.
@bengotow Sorry to bother the author with this request again, but it seems that the capability to choose the install path of mailspring is not achieved yet. Is it still in the plan or no longer considered?