Syntaxis's page

I'm not having trouble understanding the rule of space and how space and movement works. That's not the issue. This is more of a logistical question that I've been struggling with trying to justify the way Pathfinder approaches the rules for how much space certain creatures occupy. For example, a 100 foot long creature somehow still only occupies a space of 30 feet, so 30 feet across and 30 feet long. I could understand this if the creature was simply massively tall or could coil up like a snake, but a big, bulky creature just would not fit in that area. Can someone please shed some light on the reasoning for this kind of thing?

I'm having some serious trouble understanding the point of table 2-1 in the Bestiary. Mathematically, it makes zero sense to me and doesn't seem to have any logic behind it. I can advance a monster just fine without it, but I want to know what it's trying to accomplish. The Hit Point Change column makes no sense, and doesn't match up to what the CRs should be for adding that much HP. The rest of the chart doesn't add up either, with no reasoning behind the seemingly arbitrary increases to AC and Damage. Either I'm an idiot or this chart is terrible. Could someone please enlighten me to the purpose of this thing?