Dani, Getting directly to the point of Why is it that your competitors 'Smaller in size' than your site are getting more 'bigger budget' advertisers than you -

It is either a case of :

a. Your competitors being at the Right Place at the Right time.

b. Your competitors making better pitch than you.

c. or, Both.

From your post I will take it that you are NOT competing Head On with you for the Advertiser. And, that brings me to the question - How are you promoting your site to advertisers? . Yes, promoting that your site is available for them to place an ad on!

In your post you have just said that you place a link to your Media Kit on the bottom of evey page. How many of your members run a business that you can target as your advertisers?

Second question - Are you dealing direct one-to-one with advertisers? Have you ever considered involving Ad Sales companies for this purpose? Most of the times these big corporations do not mess their hands in dealing with one or two publishers [ Unless you are someone of the size of NewsCorp ]