I may be wrong but I think a simple misinterputation of the diagram is happening. The Wikipedia document states "it can be seen that a down stream low pressure (shown by the gradient lines)". So the pitch of these blue/green gradient lines show the pressure level normalized to ambient condition. The negative slope gradient lines are downstream and to the left of the venturi. Taking this in mind the media flow direction is from right to left. So the venturi inlet is of a larger diameter than the outlet.