Does the chart on the main page when you click into Marketing Activities show known, anonymous, or all (both) types of leads?
I'm just wondering as the rate of growth is quite high and I'm already trying to find ways to keep within my licensed limits!
It's based on the last 3 months of net new Known leads.
You should check your Lead Database for an accurate count though. Marketo tends to only ask for more money at renewal time unless you are really, really off your tier.