IMatch performs many things in the background, especially when the user is doing nothing and the system is "idle".
Collection updates, index updates, calculating the contents of data-driven categories, database optimizations. Basically everything that gets in the way.
It also performs folder scans when it receives notice from Windows about changes made to folders you have indexed in IMatch. And since Windows also does work when the user is idle (e.g. file system updates, defrag, thumbnail database updates for Explorer) this can happen fairly often.
You have set the log to debug mode (which is good for a Beta). This makes IMatch write very much information, even for repeating procedures like checking queues, memory state and whatnot. During your idle period, the folder
H:/xbeb/D/Canon/EOS/2010/09/2010-09-22/
was updated, so IMatch checked for new, removed and updated files. This causes all kinds of other activity, like collection updates, category updates and similar. This explains why our log file is growing.