Yes. When you share a folder into someone elses file space, that folder still belongs to the original person and thus is subject to the original persons quota.
FYI sharing is the reason that your folder path looks like:
/username.domain.tld/files/
If
joeblogs@fastmail.fm shares his folder xyz with you, then you see:
/username.domain.tld/files/
/joeblogs.fastmail.fm/files/xyz
The hope was one do to add email maybe into there as well:
/username.domain.tld/files/
/username.domain.tld/email/
Anyway, trying to think ahead a bit...
Rob