How old are you? Are you planing? I assume you are planing or getting into the straps would be impossible. Since you only weigh 43 kg, I would guess that the foot straps are pretty far apart for your size, plus it's a long reach back to the back strap.
Again, I think you may need more speed on the smaller board, and you have to use the boom / harness and front foot to hold almost all of your weight when you move your back foot. If you can do it on the larger board, I don't see why you can't on the smaller board, if you have enough speed.
Also, with your small sails, you also have short booms, which requires your body to be closer to the mast, which makes the back foot strap far away. You may try moving your mast base back some to shorten the distance to the back strap.