HI Yukari Kamata 

I dont think the size of the TM can directly relate to the number of TUs.

Example you could have large TU segments that comprise of long words, many characters and tag management. 
Certain features like uplift may also expand the size of your database - despite the TU count remains the same.
Your TM database could also have lots of custom filters and custom language resources it needs to manage.

The important this to ensure is: 

1. Ensure your TM has been upgraded: https://docs.rws.com/980998/534257/trados-studio-2022/upgrading-file-based-tms-from-navigation
   
2. Keep it well maintained by following these articles https://docs.rws.com/980998/361179/trados-studio-2022/fine-tuning-performance-settings 
3. Confirm if you can export the SDLTM data to TMX
4. If need be, try repair your TM: https://gateway.sdl.com/apex/communityknowledge?articleName=000008078

I hope this gives you peace of mind

Lyds

Thank you for your reply to my question.

I understood there was no relation between TM size and the number of TUs.

I confirmed/tried four items 1. to 4., but I couldn’t resolve this problem.

1. The TM has already been upgraded. Even if I upgrade the TM, there is no change.
2. I confirmed this menu, but it looked there was no problem.
3. I can export the TM to TMX file, and the contents are the same as the TM.
4. I tried the tool, but it does not have effects.

I’ve decided to save backups of TM data frequently.