Well, basically, max manpower is a number that is derived from the amount of soldiers you have in your nation. The total amount of soldiers is the sum of the sizes of the individual soldier POP's you have.
As all POP's grow slowly over time, so do your soldier POP's, and thus the max. manpower they produce collectively.
As for the mobilisation. Divisions in the mobilisation pool aren't really divisions, unless you actually hit the mobilize button. They are reservations, paid for with canned food and small arms, to conscript worker POP's (like farmers, laborers, crafsmen and clerks) into soldiers during a time of crisis. Unless you actually mobilise, it's just that: reservations. After mobilisation, enough worker POP's are converted to soldier POP's on a temporary basis to accomodatre the amount of dvivision you have made reservations for. As you now have more soldier POP's, you now also have the max. manpower to accomodate them. This continues until you hit the demobilise button. That converts the temporary soldier POP's back to their original type, and sends them back to work. Consequently, your max manpower will drop as well.
Jan Peter