This is a long standing bug that involves (as far as I can tell) OpenTk and MonoGame. Probably the best solution, for the time being, is to set the Window.AllowUserResizing property like so:
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
graphics.PreferredBackBufferWidth = 800;
graphics.PreferredBackBufferHeight = 800;
this.Window.AllowUserResizing = true;
}
See this GitHub thread for a related discussion:
https://github.com/mono/MonoGame/issues/628[
^]