All of this is stored in a database, which does have its size constraints. Which is why there's no "General" forum here and the site is set up to prune/delete topics that haven't been viewed for the last 20-30 days. They won't get deleted if a moderator makes them a "sticky", or if their popular.
This is also why we don't allow uploading of images or files to the site, including avatars. It eats up site disk space and bandwidth quickly, and there's plenty of free file/image hosting sites out there that can be used to link to.