You could use Warpriest's sacred weapon damage if you take Weapon Focus. I suppose you could dip Wizard or Ranger or just UMD a wand of Gravity Bow. That's about all I can think of. It will lock you into a specific class, but it should stack with Gravity Bow.
It's important to note that using called is not a free aciton. It's a swift action.
A slotless continuous item of reduce person would be a custom item that would need to be approved by your GM. And beyond that if you're really set on using a crossbow, I would highly suggest you use a light crossbow instead of a heavy crossbow, because it will allow you to make iterative attacks. Which will be more effective in the long run than a single high damage dice attack which you can only make once a round.
Sure, eventually you could reload the heavy crossbow as a free action with crossbow mastery. But it's an extra feat to spend, and the damage difference for the base weapons is 1d8 vs 1d10. When enlarged it's 2d6 vs 2d8. A difference of two points of damage but requires the expenditure of a feat, a feat which can probably be spent on something more advantageous.
If you're playing a ranged attacker who is doing something other full attacking versus a standard action attack you're setting yourself up for failure.
Note that the crossbow enlarges with you if you are holding it when enlarge person activates (and remains Large size as long as stays in your possession during the effect). You only need to purchase Large ammunition, as items leaving your possession are the only ones that return to their "original size."
This means that you can carry Medium sized ammunition for when you are not enlarged and only need to have one crossbow.
If you really want to focus on crossbows I highly recommend you look at the boltace gunslinger archetype. It makes them significantly better. However, this doesn't solve the mobility issue you seem to be conerned with. While you may want to be mobile the only way to deal significant damage with ranged attacks is to make lots of them.
Your crossbow doesn't get to add strength to damage, so you're looking at enhancement bonus, deadly aim, and fighter training to increase your damage. All these things scale slowly. Even vital strike and it's follow ups can only help so much. It will never be "big damage" compared to a full attack.
The best option overall, if you want mobility and to full attack, is to play a ranger. You get an animal companion at level 4, and you ride them. You can make full attacks at no penalty so long as your mount moves only a single move action (instead of using both the move and standard action to move). Even if the moount does double move or run, it is a penalty to attack, but you can still full attack. Mounted archery is the bees knees. And mounted crossbows going to benefit the same way.
Mobility + full attack = winning