The answer is obvious: you add more sand...(to get to the next level). Sorry, I just had to, and humor is healthy right? :-)
On a more serious level:
1. The fact that you're doing lots of small jobs is already a good start (especially the "lots" part), because if you do a good job, clients will remember you, and one day, when they need a bigger job, you'll be the one they turn to.
2. Partner with someone to create a win-win situation. For example: when contractors build large housing projects, they often work with 1-2 large companies that design kitchens, and offer their services when they sell the houses to the buyers (before they're even built). These kitchen companies pay a percentage of their profits, or a one time fee, for this partnership. You could do the same: the contractor would offer the buyers your landscaping services (it's great moving in to a new house with an amazing garden) and the contractor would get a commission for each paying client.
3. One big project: big jobs require big properties and clients with money. Focus on getting one big client, offer him the project at almost nothing*, and then over deliver (do an amazing job). The discounted rate would be on the condition that they (1) give you a recommendation, (2) let you photograph the project and (3) recommend you to their friends. Just the fact that you completed such a project, will be of value when you pitch to other similar status clients.
4. Online targeted advertising - create a well designed website (you can do this for a rather cheap price on Fiverr), add pictures of your best projects, and advertise to your target market. Try be original: example: go to a place that needs some work done (perhaps a neglected public area), and create something amazing - and then leave a funny sign with your details. You might even get some media attention (just be sure you're not doing anything illegal).
I've successfully helped over 300 entrepreneurs and businesses and I'd be happy to discuss other marketing methods if you need to.
Good luck