System storage really big

Why are you sure it isn't normal? If you are then why are you asking here?

It is pretty normal for a well-configured system inclusive of all needs and user data will typically use up 50-60GBs of space. My system uses 139GBs.

Sort all files by size and delete old large files.

Open Finder, go to Recents, and look at the Size column. Click on it to sort all recent files by file size (from large to small). You'll have a clear overview of what items are eating up a large amount of space, e.g. From 1 GB to 10 GB, and from 100 MB to 1 GB.

In my MacBook Pro, I found a few large videos that could be transferred to an external drive.

See if this help explain things.

Disk Space - Other in High Sierra, this is now included in System

Disk Space - "Other" and What Can I Do About It?

That is what I'm referring to.