For those of you did not had a look at the LogFiles: using Ajax.NET or any other AJAX library will increase the HTTP requests made to the server. If you have enabled the log files you will get very large files.
To disable the log file for Ajax.NET requests you have to create a folder with the same name you configured the httpHandler (see web.config). By default the folder is named ajax. To disable logging for that folder you have to go to the properties of the folder by right-click in IIS manager on this folder. Disable there logging of this folder: