If you're getting a lot of small jobs - congrats! That means you have a lot of customers who know your value and are willing to hire you.
That said, I TOTALLY get the desire to want to have fewer clients to manage, at a higher price point per project.
The first thing I'd have you ask yourself is this: Who are the people who would hire you for the kind of bigger jobs you're after?
Are they homeowners in more affluent neighborhoods? Development companies building new neighborhoods? Businesses who want their grounds to look nice for visitors?
Identify who they are, and put yourself in the same rooms as them. Find the kind of events they go to, and go to them to. Print of business cards that are more focused on the longer-term projects you'd like to have, rather than the smaller, one-off projects you're currently getting.
And leverage your testimonials. If you're continually landing lots of jobs, that tells me that people are happy with your work. If you don't yet have testimonials, ask your previous clients to give you some. Putting these on your website or on your other printed marketing material can go a LONG way in showing a new target market you're trying to break into that you're trustworthy and someone they'll want to hire.