Since a long time though; This isn't anything new to WLM...
However, there is nothing much you can do except for either splitting up the file by hand (notepad, etc), or moving the file to another (archive) location or following the instructions given by messenger.
The larger log files get, the longer it takes for certain functions to function (for starters logging itself of course, also reading/showing the previous conversations in a chat window, etc). And the XML logs can get pretty big.... So, although annoying, it is also neccesairly...
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