The Citizens United campaign decision in the US helped lay the groundwork for the proliferation of Super PACs, and is widely credited with increasing the spending gap between wealthy donors and the average citizen.
It's certainly easy to see why it wouldn't increase expenditures by less wealthy individuals: an individual donation limit around $5000 means that only the relatively wealthy can afford to spend the full amount, let alone on multiple elections, let alone seeing that amount of money as disposable.
However, it's less clear why it would notably increase the amount of campaign spending by wealthy donors. Individuals could spend unlimited money on personal advertisements before the decision, and super wealthy donors seemingly already had enough money to easily fund their own advertisements.
However, some research suggests that it increased expenditures by wealthy individuals significantly. Is this the case? And if so, why?