похоже это viewstate. избавиться от него можно так:

Save server control view state only when necessary.
Automatic view-state management is a feature of server controls that enables them to repopulate their property values on a round trip (without you having to write any code). This feature does impact performance, however, since a server control's view state is passed to and from the server in a hidden form field. You should be aware of when view state helps you and when it hinders your page's performance. For example, if you are binding a server control to data on every round trip, the saved view state is replaced with new values that are obtained from the data-binding operation. In this case, disabling view state saves processing time.
View state is enabled for all server controls by default. To disable it, set the EnableViewState property of the control to false, as in the following DataGrid server control example.

<asp:datagrid EnableViewState="false" datasource="..." runat="server"/> --- для своего dropdownlist
You can also disable view state for an entire page by using the @ Page directive. This is useful when you don't post back to the server from a page:

<%@ Page EnableViewState="false" %>
Note The EnableViewState attribute is also supported in the @ Control directive, which allows you to control whether view state is enabled for a user control.
To analyze the amount of view state used by the server controls on your page, enable tracing for the page (by including a trace="true" attribute in the @ Page directive) and look at the Viewstate column of the Control Hierarchy table. For information about tracing and how to enable it, see ASP.NET Trace.

также в MSDN есть интересная статья Developing High-Performance ASP.NET Applications [C#] ищется по view states.