[Brian]

What do you have selected in the "Export Area" pop-up?

From a few minutes of fiddling around, it looks like "Current Selection and "All Objects" will defer to the shape size when exporting. If a shape is larger than the canvas, those extra pixels get exported.

Also, keep in mind that a stroke on a shape is centered on the border of the shape. If you really pump the stroke width on a shape, you can see how the stroke may cause the exported png to be larger than the size of the shape.

In case it helps: if you choose "Current Canvas" (or "Entire Document", I suppose, in a 1-canvas document) the app will clip the export right at the canvas.