Weird error when trying to make request to web-service with large argument

Hi!
I'm trying to call web-service' method from JavaScript passing large argument(150kb string)
Getting alert with following message: "The server method 'MethodName' failed with the following error: System.ArgumentException-- Maximum length exeeded. Parameter name: input"
though is stated in web.config:

<httpRuntime maxRequestLength="1048576"/>
Have you any ideas why does it happen and how to fix it?
Thanks,
Dan

...