Yep, The_Dog's got it right. If you reversed the ground & welded areas you'd move the big end of the conrod closer to the crank's centre of rotation and shorten the stroke.
Of course the crank I showed is a very simplified one and the design is taken from a typical car's crank so in reality it'd probably look somewhat different but the theory's still the same. You also need to take things like the piston's rise into account, and find custom pistons where the gudgeon pin is set higher into the piston body so crown doesn't run into the valves etc.
Another way to have the same effect is to use conrods with a smaller diameter big-end journal. This allows you to simply re-grind the journal on the crank with a different centre, without the need to weld. This used to be preferred method on the old 1960's & 1970's Mini's, which originally came with a "large journal crank" but later came out with a "small journal crank". The real trick these days is to find one of the early "large" cranks that's been unmolested and is in good condition. Very useful as they're a cast iron crank and very difficult to weld.
