well it shouldn't - however I think a small site done badly is much easier to fix, whereas a large site done badly is much more difficult to fix - if it was 9000 similar pages (i.e. used same content type / layout) then it wouldn't matter one bit - but the situation we have is that site editors are uploading content and combining templates in ways that are hard to predict - and if you make changes to the CSS unlike a 50 page website it's a collosal time consuming job to check the visual impact accross 9000 pages, it's going to take half a day and be a pretty unpleasant job.
It's not just a matter of 9000 pages, that's just an indicator of scale, just more of a combinatory problem, more content, more templates, more editors, more devs, more designers.
Smaccs works as well for small as well as large sites, it's just that smacss etc. is a guideline for architecting the CSS it doesn't do the hard work of solving the problem, and there are easier ways to do CSS, if you don't need the CSS to be scalable and modular then why waste your time making it that way? If you're site is small, the pages are very similar to each other, or it's lifespan is short (i.e. unlikely to need a lot of maintenance) it might not be worth the trade-off