In the picture of this head you can clearly see the valves are taking up all available chamber space. The reason being i machined the head to accommodate +1mm valves all round thus increasing the amount that the overbore pistons can draw in & in the real world increasing the amount ram air can push in. Then because you successfully squeezed more in, the compression pressure has actually increased. Because you increased the in volume its as important on this particular model to increase the out capability.
Good porting is crucial to maximising velocity, the faster it goes in = more goes in, there is a fine balance optimising flow. Seen here is the cut back valve guides that present less resistance to exhaust gases velocity thee faster it comes out = the more comes out.