If you are importing into the library instead of “leave files in place”, that can take a lot of additional room.
If your import preferences automatically create proxies or optimized media, that also will take lots of room. If the original media is H.264, proxies add roughly 60% to that size and optimized media is about 6x that size. FCPX tries to predict the space needed for those and if insufficient to complete the task, will refuse to start with an “insufficient space” message.
If that’s not it, run Disk Utility First Aid on all volumes, then try again.
Here is a free app called OmniDiskSweeper that can help track down space consumption:
https://www.omnigroup.com/more/
If you have several libraries, Final Cut Library Manager can help safely free up space:
https://www.arcticwhiteness.com/finalcutlibrarymanager/