Why db2data folder changes its size (increasing and decreasing )? we have DB2 9.7. It has almost 30 GB.
Data are on this /opt/IBM/db2data/ctginst1/NODE0000/DB_NAME/Txxxxxx

I noticed that every day during the day (suppose because of the workload) DB change its size (partition /opt/IBM/db2data)
For example used 26 GB, 27 GB, 28 GB, then it returns to 26GB.

I noticed that on Linux when using 
df -h.
Available space grows and then returns to old value.
How is this possible? Is this regular behavior? Also only one TABLESPACE has over 24 GB. Others are smaller. Shouldn't they be splatted equally or not?

Data are not deleted from database so that is not the reason why DB increase/decrease!
Temporary tablespaces grow and shrink as required. They can be used for temporary tables, sorts, or certain types of joins. I still think you should start reading manuals.