Having experience of heavily trafficked systems (1000 per day) with wild swings of influence both during conflict and peacetime I would have to disagree in relation to a hard cap. Traffic in the thousands with the associated activity can cause dramatic swings. CG related systems often suffer from it, as do massacre/skimmer grinding systems. This is most readily apparent when a faction state activates that negates a particular activity (e.g. Election negating BH effects). If you're afflicted with traffic, pray its working for you!
There is a practical limit to how much a single cmdr/small group can do in the space of a tick, so in operational terms the above distinction is largely meaningless!
It is the case that the larger the population the harder it is to shift and requires significant ongoing effort to reach a particular goal.
edit: I would have to double check our records but if I recall correctly I saw a drop of at least 25%.
Edit 2: Goeman's post above may also account for such changes as an alternative theory.