Why are my applications taking up so much storage on my MacBook?

Make sure to regular Empty the trash in the Trash bin on the Dock

Restart in Safe Mode. This will perform a Disk Repair, clear system cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

EDITED - Added info.

One thing to consider is that information is not only the size of the app itself, but also all of its support data and data you created with the app. My only example to offer is GarageBand and all of its synthesized loops.

However, that number seems awfully high, so I agree that reindexing Spotlight might help reduce that number. All of those storage numbers are calculated by Spotlight grouping items into categories and summing up the storage used. If Spotlight index is not working, the numbers can be off.