migrating 100gb system

What and how did you migrate? Maybe you copied over all your user data too. Where did you read it took up 100GB of space? Some of the reports on space usage are not accurate.

Run

OmniDiskSweeper

"The simple, fast way to save disk space"

OmniDiskSweeper is now free!

OmniDiskSweeper and OmniWeb - The Omni Group

This will give you a list of files and folders sorted by size. Go after things you know that are big.

To see all files, you need to logon to each user on your system and run OmniDiskSweeper.

Because TimeMachine stores multiple configurations of your system. That's why the TimeMachine backup on your external drive can be up to 5 times larger than than your actual.

FYI: It's "GB," not "gb."

G=giga B=Byte

g=grams or the acceleration of gravity b=bit

Your question makes a lot of difference if the correct terminology/acronym is not used. So the solution may be completely different or inappropriate based on the terminology.

Gb = gigabit

GB = gigabyte

gb is meaningless

I think Lanny made the correct assumption but was only pointing out the difference to help you in future discussions.

Look for old Time Machine Snapshots on your computer and delete them.