You're probably exceeding the maximum response length:
maxJsonLength
Specifies the maximum length of the JSON string (the maximum number of UTF-8 characters). The default length is 102400.
Change the settings in your
web.config
file to increase the maximum:
<configuration>
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="500000"/>
</webServices>
</scripting>
</system.web.extensions>
</configuration>