Marvel and DC books sell better as single issues rather than collected editions, yet include extra stuff in the collections; Image books on the otherhand generally sell better in collections but put all their bonuses in the single issues. Why?

Someone who runs a comic shop could probably answer the sales questions much better than me.but I would suspect that the M/DC single issues sell better than their trades partly because that’s how those books have traditionally sold, where the modern run of Image books are a much newer market (post the arrival of Walking Dead or Saga) and the creative teams are more likely to stay on one book together for a shorter series.– I assume they would just read better in trade because of that. as opposed to an xmen series that never really ends and might have lots of fill in people in a trade, 

The only recent corporate trade I have here is a 2014 Harley quinn book – it has no back up in it just 3 pages of ads at the end.

As far as the back up stuff at Image, I guess that depends on what Image book– With Image what you put in your pages is up to you soo I assume more back up stuff would go in just because the standard issue length is printed at 28 pages but most artists do more like 20-24 pages of comics in that. Where the corporate comics have i assume a set # of ads to run–So I assume they have less space to run extra pages in issues?