A move action simply does not take as long as a standard action. You can turn a standard action into a move action, simply giving up however much extra time the standard action would have taken, but you cannot do the reverse because it would take more time.
How much time is explicitly left undefined. The whole round is roughly six seconds, with everyone acting near-simultaneously (in-character, people are not waiting their turn), but no further breakdown is given aside from Standard > Move > Swift > Free > Non. Note you cannot trade a standard or a move action for a swift the way you can with standard to move, however. For that one, the reason is just “because the rules say so” and because a lot of things are balanced by there only being one swift action per round.
So you cannot choose to not move (trading in your move action) in order to attack (a standard action) or use a breath weapon (also a standard action). Note that most spells are a standard action; a wizard getting a pair of spells per round just because he hasn’t moved would be insane, and even more overpowered than the wizard already is.