That's not the same thing as a temporary spell effect increasing your size, though. Enlarge Person specifically increases the size of your weapon. Alter Self doesn't spell it out so clearly, but says if the new form is capable of wearing/using the item, it can stay (ie, creature shape, possession of or lack of hands, etc...). If the original size of the item mattered, you'd think they'd have added some line about "size permitting," too. Enlarge sets the precedent that size altering magic affects all your gear, including weapons, as long as they remain in your possession. Other spells, like invisibility, further confirm this.