Gmail has stopped its evergrowing email storage space.
What was the storage space counter at before the move to 15GB constant? (Doesn't have to be exact)
According to the official announcement for the new sharing of storage, Gmail had a 10GB limit while Google+ Photos and Google Drive shared 5GB before the change. (Confirmed on this other blog post which shows a timeline where the Gmail upper limit was expanded to 10GB on or about April 24, 2012.)
Whether that's what was displayed in the "evergrowing counter" is unknown.