Due to the massive size of Wikipedia and sister projects, even apparently insignificant changes applied to a large number of pages can affect a significant number of viewers. I believe that such changes should be announced at the very least to the Technical Ambassadors so they can respond to questions. While it is difficult to say what "large number of pages" should mean, I would include here changes that affect common page elements (TOC, tables, images, infoboxes)
Topic on Talk:Technical Collaboration Guidance/Milestone communication
All changes might effect large number of pages. I think the point of concern is the level of impact (iow: how much is changing). We're a big website with tons of pages and tons of viewers. So any absolute (or even relative) metric for "large" wouldn't be useful in gauging importance for further review/announcement.