[Christ]

You raise some interesting points, but given the images I've provided, some of the answers to them are already present.

To being "less compact": That would be the primary reason for shortening the flow tract. Compare the sizes of parts from one drawing to another, they're nearly identical. This means that the valve and guide are the same length, or at least nearly the same length.

Springs could be seated in depth of the red shaded area, which would require them to be wider, but not taller in any way. Obviously, the "package" has grown headwise, but the runner has shrunk to compensate. The rockers might be pushed higher compared to the initial placement, but the head is shorter, so a taller rocker cover would only take up space that was gained by shrinking the head casting.

I'm not saying I have all the answers here, nor do I have real-world data or experience to back up what I'm saying, but the ideas seem to work on paper. (or MSPaint, rather.)