The leaf pile will only get bigger if you let the Sims continue to rake. There's no option on the leaf pile for you to add more, it's simply done by clicking the ground near it where there are leaves and raking. Once it's the size of the hay stack from Pets is when you'll have the option to "woohoo" with someone you're romantic with on the lot.
Warning: Your Sims hygiene will TANK. So as nifty as it seems, it always seemed to just drag the hygiene bar to it's bottom (especially if your Sim had just finished raking which also costs a lot of hygiene to do).