One of the problems of big ActiveRecord models is their low cohesion. In Rails we group behaviour around the entities of the domain we're modelling. If we use only ActiveRecord models for that, we'll probably end up with classes full of actions, in most cases, completely unrelated to each other (except for the fact that they act on the same domain entity).
To illustrate the issue let's imagine a big Post AR model of a blog application. In this app, users can add tags to their posts. Since this is the only class with this ability, I decide to put the related logic in the Post model itself. For this, presumably at the beginning of the file, I write a couple of has_many entries for tags and taggings. Then, fifty lines below, along with the rest of scopes, I add one more to find posts by tag name. A couple of hundred lines later, I put a virtual attribute tag_list which will be used to update the associations from a string of tag names separated by commas. Fin
