I think most large sites operate under the premise that you SHOULD link to them because they are important, and that linking back to you is not a priority...I mean, they are important, you are not, why should they send surfers there?

It's also possible that the lack of response is calculated so that they get the link pointed at them, the smaller site tries for a while to get the link back, after a while forgetting the effort and also forgetting to remove the link to the larger site.

JMHO