True change is gradual. A plan that is too ambitious often results in a mind set in opposition to nature, a war pitting gardener against pests, weeds, and unyielding earth. Ideals tend to get lost in the heat of those battles. When overwhelmed, you might consider chemical reinforcements, thinking that the means may appear to justify the end. That's not true.
Gardening is all about means, about process, about working with the earth.
There is no end. By beginning small you allow yourself time to gauge what your garden will require from you. Remember you will be changing too, altering your own routines to make room in your life for a garden. If you dig up more space than you actually plant, the bare earth will soon be covered with weeds, and you will be mired in frustration. Starting small allows you to become intimately acquainted with the plants you are growing and increases, rather than diminishes, your rewards.