You clear a few gigabytes, carry on working, and a day or two later the drive is full again. The useful question is not just what is large. It is what changed.
Start with the timing
A drive that grows once after a Windows update is a different problem from one that loses 3 GB every afternoon. Try to pin down when the free space changes. Does it happen after a backup, a game update, a database job, a build, or simply after the computer has been running for a few hours?
If the drive is already nearly full, do one scan now. SizeTrend's Overview pane can call out large applications, old Downloads, Windows and browser caches, Git repositories, SQL Server storage, IIS logs, virtual machines, and other recognizable consumers. That often gives you a useful first lead.
The best evidence is two scans
A list of today's largest folders may not reveal the cause. A 40 GB folder that has been the same size for years is probably not responsible for the 6 GB you lost last night.
Scan the affected drive while you still have a reasonable amount of free space.
Use Filter | Compared to Scan | Save Current Scan to keep that point in time.
Wait until the unexplained loss happens again, then refresh the scan or start SizeTrend again.
Choose the saved scan under Filter | Compared to Scan | Previously Saved Scans.
The result contains the difference between the two scans. Folders that did not change drop out of the way, so the new log, cache, backup, temporary file, or virtual disk has a much harder time hiding.
Saved-scan comparison is available during the 14-day Professional evaluation and with a Professional license. After the evaluation, Free can use one automatic What Changed comparison when a compatible automatic snapshot is available.
If you do not have an older scan
Use Modified in past... and Created in past... to narrow the current scan to a useful time range. A seven-day filter is a good place to start if the trouble began this week. Sort or browse the remaining folders by size and follow the growth down to the actual files.
Dates are clues, not proof. Some installers preserve original file dates, and a growing database file may have been created years ago. When the date filter does not explain the missing space, save the current scan and wait for the next occurrence.
Do not clean up the symptom blindly
Once you know which files grew, identify the program or Windows component that owns them. A browser cache, an IIS log, a SQL Server transaction log, and a virtual machine disk all need different treatment. Deleting the largest file in Explorer can break an application or make recovery harder.
SizeTrend helps you get from "the drive is full" to a specific location and time period. It does not assume that every old or large file is disposable. Use the program's own cleanup or administration tools whenever the data is managed by an application or by Windows.
Catch the next unexplained change.
Download SizeTrend, save a scan, and compare it after the drive loses space again.