PSTCompact - Size Does Matter. Reduce the size of your Microsoft Outlook PST Files with PSTCompact, the PST File Optimizer. It is a server-based installation that is easy to install, easy to use, and will reduce backup time. Using Microsoft Outlook will very quickly result in Personal folders that grow much faster than you can imagine. PST files between 1GB and 2GB are more a rule than an exception. Resulting in slower and worse case even unusable e-mail software. Why do PST files only grow and never reduce in size? PST files are not designed to do intelligent file management. Like most databases and file systems, deleted space is only fully recovered when you run special programs to do so. PST files will keep eating your disk space. It does not matter if you delete old emails or export your large attachments. However, the PST compression will not happen automatically. The solution for automatic PST compression and a healthy, slim and fast performing PST file is PSTCompact.