System Space Hog

I have three Macs, and have virtually the same software on all three of them. On my work laptop, I seem to be gobbling up a lot of hard drive space, and at the moment I'm down to just 38.86 GB of my 500 GB HD, and I can't figure out what is eating up all the space.

I'm a web developer, and every day I download a database backup and deploy the database locally. Each of those DB files is 4GB+, and I open them to make a quick edit in TextEdit, and then run a few commands through Terminal to load the database. And then I delete the file and empty my trash. Every day that stat of available HD space continues to decrease. Just a few weeks ago, I deleted some other large files and unused programs, and got up to 115 GB of available space! Since then, I haven't loaded any new programs or downloaded any other large files. My iMac has pretty much all the same programs and many MORE media files still has 330GB+/500GB. So I believe the process I described earlier in this paragraph is creating some temp files or logs that I simply need to find and delete.

I've attached some screenshots of what I'm seeing regarding my filespace; two from built-in Mac diagnostics, and a scan from OmniDiskSweeper that shows that I'm currently only using 96GB of storage on this computer. What / where are all the other files that are taking up so much of my System space?