This is much easier to explain with the aid of diagrams (particularly Vg diagram) but I'll give it a shot..

Look in your manuals (or on the link below, Figure 4) for the diagram showing the relationship between level turn bank angle and stall speed. You will see that the steeper the bank is, the higher the stall speed is. Why? Because the more we bank in a level turn, the more we need to increase back pressure to maintain altitude. The more we increase back pressure, the higher the load factor (G-force).

Load factor is simply a number telling us how much heavier the airplane is than it would be in normal straight and level flight (2g means a 2,000lb airplane now "weighs" 4,000lb). So we can see that as the load factor (ie weight!) increases, so does the stall speed! (Perfectly logical as we already know that in level flight lift = weight, so the heavier the aircraft, the more lift is required, which is obtained by more speed!).

Va is simply the highest speed we can fly at and still guarantee structural integrity: the last thing we want to do is overstress the airframe, so we fly at a speed which will ensure that if we should encounter turbulent weather and perhaps severe up and down drafts, leading to extreme load factors, the airplane will stall before it breaks. Va is that speed, and as we've already shown, stall speed increases with weight!

If you have a C172 manual, also take a look at the weight and balance limits for normal versus utility category operations. you'll see that max weight X max load factor both come to the same total weight - the weight the wings are built to support before failing (3.8 X MGW in normal and 4.2 X MGW in utility if I remember correctly - it's been a few years since I instructed and I don't have my referances to hand!).

Hope that helps..