Tips on cutting curves with a jigsaw
Cutting curves is more difficult because the toothless back of the blade presses against the edge of the cut, forcing the toothed edge to cut sideways. The wider the blade in relation to its thickness, the greater the issue. Of course, a narrow blade is weaker.

To make it easier, there are commercial circle-cutting attachments available for sabre saws and jig saws, or you can make your own attachment.